Input/output device and electronic device

ABSTRACT

An input/output device is provided. The input/output device includes a first pixel electrode, a second pixel electrode, a first common electrode, a second common electrode, a liquid crystal, a first insulating film, a second insulating film, and a transistor. The first common electrode can serve as one electrode of a sensor element. The second common electrode can serve as the other electrode of the sensor element. The transistor includes a first gate, a second gate, and a semiconductor layer. The pixel electrode, the common electrodes, and the second gate are positioned on different planes. The second gate contains one or more kinds of metal elements included in the semiconductor layer. The second gate, the pixel electrode, and the common electrodes preferably contain one or more kinds of metal elements included in the semiconductor layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to an input/outputdevice and an electronic device.

Note that one embodiment of the present invention is not limited to theabove technical field. Specifically, examples of the technical field ofone embodiment of the present invention disclosed in this specificationand the like include a semiconductor device, a display device, alight-emitting device, a power storage device, a memory device, anelectronic device, a lighting device, an input device (e.g., a touchsensor), an output device, an input/output device (e.g., a touch panel),a method for driving any of them, and a method for manufacturing any ofthem.

2. Description of the Related Art

Transistors used for most flat panel displays typified by a liquidcrystal display device and a light-emitting display device are formedusing silicon semiconductors such as amorphous silicon, single crystalsilicon, and polycrystalline silicon provided over glass substrates.Furthermore, such a transistor employing such a silicon semiconductor isused in integrated circuits (ICs) and the like.

In recent years, attention has been drawn to a technique in which,instead of a silicon semiconductor, a metal oxide exhibitingsemiconductor characteristics is used in transistors. Note that in thisspecification, a metal oxide exhibiting semiconductor characteristics isreferred to as an oxide semiconductor. For example, in Patent Documents1 and 2, a technique is disclosed in which a transistor is manufacturedusing zinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductorand the transistor is used as a switching element or the like of a pixelof a display device.

What is desirable is a touch panel in which a display device is providedwith a function of inputting data with a finger or the like touching ascreen as a user interface.

A display device provided with a touch sensor or a display moduleprovided with a touch sensor is called a touch panel, a touch screen, orthe like. Furthermore, a device that has a touch sensor and does nothave a display element is called a touch panel in some cases. Thus, adisplay device provided with a touch sensor or a display module providedwith a touch sensor is called a display device having a touch sensor, adisplay device having a touch panel, a touch sensor having a displaydevice, or a touch panel having a display device in some cases. Notethat a display device provided with a touch sensor is referred to as atouch panel.

For example, Patent Documents 3 to 6 each disclose a touch panel using aliquid crystal element as a display element.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2007-123861

[Patent Document 2] Japanese Published Patent Application No.2007-096055

[Patent Document 3] Japanese Published Patent Application No.2011-197685

[Patent Document 4] Japanese Published Patent Application No. 2014-44537

[Patent Document 5] Japanese Published Patent Application No.2014-178847

[Patent Document 6] United States Patent Application Publication No.2008/0158183

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide athin input/output device. Another object of one embodiment of thepresent invention is to provide a lightweight input/output device.Another object of one embodiment of the present invention is to providean input/output device with a small number of components.

Another object of one embodiment of the present invention is to providea highly reliable input/output device. Another object of one embodimentof the present invention is to provide an input/output device with highdetection sensitivity. Another object of one embodiment of the presentinvention is to provide a novel input/output device.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects can be derived fromthe description of the specification, the drawings, the claims, and thelike.

One embodiment of the present invention is an input/output deviceincluding a first pixel electrode, a second pixel electrode, a firstcommon electrode, a second common electrode, a liquid crystal, a firstinsulating film, a second insulating film, and a transistor. The firstcommon electrode can serve as one electrode of a sensor element. Thesecond common electrode can serve as the other electrode of the sensorelement. The transistor includes a first gate, a second gate, and asemiconductor layer. The semiconductor layer includes an oxidesemiconductor in a channel formation region. The second gate includes anoxide conductor. The oxide conductor contains one or more kinds of metalelements included in the oxide semiconductor. The semiconductor layer isover the first gate. The second gate is over the semiconductor layer.The first insulating film is over the second gate. The first pixelelectrode, the second pixel electrode, the first common electrode, andthe second common electrode are over the first insulating film. Thefirst pixel electrode partly overlaps with the first common electrodewith the second insulating film interposed therebetween. The secondpixel electrode partly overlaps with the second common electrode withthe second insulating film interposed therebetween. The liquid crystalis over the first pixel electrode, the second pixel electrode, the firstcommon electrode, and the second common electrode. The first pixelelectrode is apart from the second pixel electrode on one plane. Thefirst common electrode is apart from the second common electrode on oneplane.

The transistor is included in at least one of a display portion or adriver circuit portion. For example, an input/output device of oneembodiment of the present invention includes two transistors that areeach the above transistor. A source or a drain of one of the twotransistors may be electrically connected to the first pixel electrode.A source or a drain of the other of the two transistors may beelectrically connected to the second pixel electrode. Alternatively, thetransistor may be positioned in the driver circuit portion.

In each of the above structures, the second gate may be electricallyconnected to the first gate.

In each of the above structures, the second insulating film may be overthe first pixel electrode and the second pixel electrode, and the firstcommon electrode and the second common electrode may be over the secondinsulating film. In each of the above structures, the second insulatingfilm may be over the first common electrode and the second commonelectrode, and the first pixel electrode and the second pixel electrodemay be over the second insulating film.

In each of the above structures, the first pixel electrode and thesecond pixel electrode may each include at least one metal elementcontained in the oxide semiconductor. Furthermore, the first commonelectrode and the second common electrode may each include at least onemetal element contained in the oxide semiconductor.

In each of the above structures, the oxide semiconductor and the oxideconductor may each include an oxide containing indium. Furthermore, thefirst pixel electrode and the second pixel electrode may each include anoxide containing indium. Furthermore, the first common electrode and thesecond common electrode may each include an oxide containing indium.

In each of the above structures, the oxide semiconductor and the oxideconductor may each include an In-M₁-Zn oxide (M₁ is Al, Ti, Ga, Y, Zr,La, Ce, Nd, Sn, or Hf). Furthermore, the first pixel electrode and thesecond pixel electrode may each include the In-M₁-Zn oxide. Furthermore,the first common electrode and the second common electrode may eachinclude the In-M₁-Zn oxide.

In each of the above structures, the first pixel electrode, the secondpixel electrode, the first common electrode, and the second commonelectrode may each have a function of transmitting visible light.

In each of the above structures, a first conductive film may bepositioned between the first insulating film and the first commonelectrode. The first conductive film may have a lower resistivity thanthe first common electrode and may be electrically connected to thefirst common electrode. Furthermore, a second conductive film may bepositioned between the first insulating film and the second commonelectrode. The second conductive film may have a lower resistivity thanthe second common electrode and may be electrically connected to thesecond common electrode. The first conductive film may be apart from thesecond conductive film on one plane.

In each of the above structures, a light-blocking film may be provided.The light-blocking film may include a region overlapping with at leastone of the first conductive film and the second conductive film with theliquid crystal positioned therebetween.

Another embodiment of the present invention is a module such as a modulein which a connector such as a flexible printed circuit (FPC) or a tapecarrier package (TCP) is attached to the input/output device or a modulein which an IC is mounted on the input/output device by a chip on glass(COG) method, a chip on film (COF) method, or the like.

Another embodiment of the present invention is an electronic deviceincluding the module and at least one of an antenna, a battery, ahousing, a speaker, a microphone, an operation switch, and an operationbutton.

According to one embodiment of the present invention, a thininput/output device can be provided. According to another embodiment ofthe present invention, a lightweight input/output device can beprovided. According to another embodiment of the present invention, aninput/output device with a small number of components can be provided.

According to one embodiment of the present invention, a highly reliableinput/output device can be provided. According to another embodiment ofthe present invention, an input/output device with high detectionsensitivity can be provided. According to another embodiment of thepresent invention, a novel input/output device or the like can beprovided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects canbe derived from the description of the specification, the drawings, theclaims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a top view and a cross-sectional view illustratingan example of an input/output device.

FIGS. 2A and 2B are cross-sectional views illustrating examples of aninput/output device.

FIGS. 3A to 3F are cross-sectional views illustrating examples of aninput/output device.

FIG. 4 is a cross-sectional view illustrating an example of aninput/output device.

FIG. 5 is a cross-sectional view illustrating an example of aninput/output device.

FIG. 6 is a cross-sectional view illustrating an example of aninput/output device.

FIGS. 7A and 7B illustrate examples of a sensor element and a pixel.

FIGS. 8A to 8E illustrate examples of operation of a sensor element anda pixel.

FIGS. 9A and 9B are top views illustrating examples of a sensor elementand a pixel.

FIG. 10 is a top view illustrating an example of a sensor element.

FIGS. 11A and 11B are top views illustrating an example of a sensorelement.

FIG. 12 is a top view illustrating an example of an input/output device.

FIG. 13 is a top view illustrating an example of an input/output device.

FIGS. 14A and 14B are top views illustrating examples of an input/outputdevice.

FIG. 15 is a block diagram illustrating an example of a touch panelmodule.

FIGS. 16A to 16C illustrate examples of a touch panel module.

FIGS. 17A to 17D are cross-sectional views illustrating an example of amethod for manufacturing a transistor and the like.

FIGS. 18A to 18C are cross-sectional views illustrating an example of amethod for manufacturing a transistor and the like.

FIGS. 19A to 19C are cross-sectional views illustrating an example of amethod for manufacturing a transistor and the like.

FIG. 20 is a cross-sectional view of an example illustrating a methodfor manufacturing a transistor and the like.

FIG. 21 is a cross-sectional view illustrating an example of atransistor.

FIGS. 22A to 22C are a top view and cross-sectional views illustratingan example of a transistor.

FIGS. 23A to 23D are cross-sectional views illustrating examples of atransistor.

FIGS. 24A and 24B show band structures.

FIGS. 25A to 25D are cross-sectional views illustrating examples of atransistor.

FIGS. 26A to 26E show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD and selected-area electrondiffraction patterns of a CAAC-OS.

FIGS. 27A to 27E show a cross-sectional TEM image and plan-view TEMimages of a CAAC-OS and images obtained through analysis thereof.

FIGS. 28A to 28D show electron diffraction patterns and across-sectional TEM image of an nc-OS.

FIGS. 29A and 29B show cross-sectional TEM images of an a-like OS.

FIG. 30 shows a change in crystal part of an In—Ga—Zn oxide induced byelectron irradiation.

FIG. 31 shows an example of a touch panel module.

FIGS. 32A to 32H illustrate examples of electronic devices.

FIGS. 33A and 33B illustrate examples of electronic devices.

FIG. 34 is a cross-sectional view illustrating an example of aninput/output device.

FIG. 35 is a photograph of an input/output device of an example.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments are described in detail with reference to drawings. Notethat the present invention is not limited to the description below, andit is easily understood by those skilled in the art that various changesand modifications can be made without departing from the spirit andscope of the present invention. Accordingly, the present inventionshould not be interpreted as being limited to the content of theembodiments below.

Note that in the structures of the invention described below, the sameportions or portions having similar functions are denoted by the samereference numerals in different drawings, and description of suchportions is not repeated. Furthermore, the same hatching pattern isapplied to portions having similar functions, and the portions are notespecially denoted by reference numerals in some cases.

In addition, the position, size, range, or the like of each structureillustrated in drawings and the like is not accurately represented insome cases for easy understanding. Therefore, the disclosed invention isnot necessarily limited to the position, the size, the range, or thelike disclosed in the drawings and the like.

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive film” can be changed into the term “conductive layer” insome cases. Also, the term “insulating layer” can be changed into theterm “insulating film” in some cases.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.In addition, the term “substantially parallel” indicates that the angleformed between two straight lines is greater than or equal to −30° andless than or equal to 30°. In addition, the term “perpendicular”indicates that the angle formed between two straight lines is greaterthan or equal to 80° and less than or equal to 100°, and accordinglyalso includes the case where the angle is greater than or equal to 85°and less than or equal to 95°. A term “substantially perpendicular”indicates that the angle formed between two straight lines is greaterthan or equal to 60° and less than or equal to 120°.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

Embodiment 1

In this embodiment, an input/output device of one embodiment of thepresent invention is described with reference to FIGS. 1A and 1B, FIGS.2A and 2B, FIGS. 3A to 3F, FIG. 4, FIG. 5, FIG. 6, FIGS. 7A and 7B,FIGS. 8A to 8E, FIGS. 9A and 9B, FIG. 10, FIGS. 1A and 11B, FIG. 12,FIG. 13, FIGS. 14A and 14B, FIG. 15, and FIGS. 16A to 16C.

The input/output device of one embodiment of the present invention is anin-cell touch panel that has a function of displaying an image andserves as a touch sensor.

There is no particular limitation on a display element included in theinput/output device of one embodiment of the present invention. As thedisplay element, a variety of display elements including a liquidcrystal element, an optical element that utilizes micro electromechanical systems (MEMS), a light-emitting element such as an organicelectroluminescent (EL) element or a light-emitting diode (LED), and anelectrophoretic element can be used.

In this embodiment, a transmissive liquid crystal display device using aliquid crystal element in a horizontal electric field mode is describedas an example.

There is no particular limitation on a sensor element included in theinput/output device of one embodiment of the present invention. Notethat a variety of sensors that can sense proximity or touch of a sensingtarget such as a finger or a stylus can be used as the sensor element.

For example, a variety of types such as a capacitive type, a resistivetype, a surface acoustic wave type, an infrared type, an optical type,and a pressure-sensitive type can be used for the sensor.

In this embodiment, an input/output device including a capacitive sensorelement is described as an example.

Examples of the capacitive sensor element are a surface capacitivesensor element and a projected capacitive sensor element. Examples ofthe projected capacitive sensor element include a self-capacitive sensorelement and a mutual capacitive sensor element. The use of a mutualcapacitive sensor element is preferable because multiple points can besensed simultaneously.

In the input/output device of one embodiment of the present invention,an electrode or the like included in a sensor element is provided onlyon a substrate supporting a display element. The input/output device ofone embodiment of the present invention is a full-in-cell touch panel.As an example of a structure of an in-cell touch panel, a structure inwhich an electrode and the like included in a sensor element areprovided on both a substrate supporting a display element and a countersubstrate or on only the counter substrate is given. As compared withthe structure, the full-in-cell touch panel is preferable because thestructure of the counter substrate can be simplified.

The input/output device of one embodiment of the present invention ispreferable because an electrode included in the display element alsoserves as an electrode included in the sensor element and thus themanufacturing process can be simplified and the manufacturing cost canbe reduced.

One embodiment of the present invention can reduce the thickness orweight of the input/output device or the number of components of theinput/output device as compared with a structure in which a displaypanel and a sensor element separately formed are attached to each otheror a structure in which a sensor element is formed on a countersubstrate side.

In the input/output device of one embodiment of the present invention,both an FPC for supplying a signal for driving a pixel and an FPC forsupplying a signal for driving a sensor element are on one substrateside. With this structure, the touch panel can be easily incorporatedinto an electronic device, and the number of components can be reduced.Note that the signal for driving a pixel and the signal for driving asensor element may be supplied by one FPC.

The structure of the input/output device of one embodiment of thepresent invention is described below.

[Cross-Sectional Structure Example 1 of Input/Output Device]

FIG. 1A is a top view of an input/output device 300. FIG. 1B is across-sectional view taken along dashed-dotted lines A-B and C-D in FIG.1A.

As illustrated in FIG. 1A, the input/output device 300 includes adisplay portion 301 and scan line driver circuits 302. The displayportion 301 includes a plurality of pixels 303, a plurality of signallines, and a plurality of scan lines, and has a function of displayingan image. Moreover, the display portion 301 also serves as an inputportion. That is, the display portion 301 includes a plurality of sensorelements that can sense touch or proximity of a sensing target to theinput/output device 300 and thus serves as a touch sensor. The scan linedriver circuit 302 has a function of outputting a scan signal to thescan lines included in the display portion 301. The pixel 303 includes aplurality of subpixels. Although FIG. 1A illustrates an example in whichthe pixel 303 includes three subpixels, one embodiment of the presentinvention is not limited to this example.

Although FIG. 1A illustrates an example in which the input/output device300 includes the scan line driver circuits, one embodiment of thepresent invention is not limited to this example. The input/outputdevice 300 that does not include any of a scan line driver circuit, asignal line driver circuit, and a sensor driver circuit may be employed,or the input/output device 300 that includes any one or more of a scanline driver circuit, a signal line driver circuit, and a sensor drivercircuit may be employed.

In the input/output device 300, an IC 268 is mounted on a substrate 211by a COG method or the like. The IC 268 includes, for example, any oneor more of a signal line driver circuit, a scan line driver circuit, anda sensor driver circuit.

An FPC 269 is electrically connected to the input/output device 300. TheIC 268 and a scan line driver circuit are supplied with a signal fromthe outside via the FPC 269. Furthermore, a signal can be output fromthe IC 268 to the outside via the FPC 269.

An IC may be mounted on the FPC 269. For example, an IC including anyone or more of a signal line driver circuit, a scan line driver circuit,and a sensor driver circuit may be mounted on the FPC 269. For example,the IC may be mounted on the FPC 269 by a COF method or a tape automatedbonding (TAB) method.

For example, the IC 268 may include a signal line driver circuit and asensor driver circuit. Alternatively, for example, the IC 268 mayinclude a signal line driver circuit and the IC mounted on the FPC 269may include a sensor driver circuit.

As illustrated in FIG. 1B, the input/output device 300 includes atransistor 201 a, a transistor 203 a, a connection portion 205 a, aliquid crystal element 207 a, and the like over the substrate 211.

FIG. 1B illustrates the cross section of one subpixel as an example ofthe display portion 301. For example, a subpixel exhibiting a red color,a subpixel exhibiting a green color, and a subpixel exhibiting a bluecolor form one pixel, and thus full-color display can be achieved in thedisplay portion 301. Note that the color exhibited by subpixels is notlimited to red, green, and blue. For example, a subpixel exhibitingwhite, yellow, magenta, cyan, or the like may be used for a pixel.

The transistors 201 a and 203 a include a gate electrode 221, aninsulating film 213, an oxide semiconductor film 223, a source electrode225 a, and a drain electrode 225 b. The transistor 201 a furtherincludes a conductive film 226, an insulating film 215, and an oxideconductor film 227. Note that the insulating film 215 can also beregarded as a component of the transistor 203 a.

The gate electrode 221 and the oxide conductor film 227 can each serveas a gate. The transistor 201 a has a structure in which an oxidesemiconductor film where a channel is formed is sandwiched between twogates. The gate electrode 221 is electrically connected to the oxideconductor film 227 through the conductive film 226. Transistors havingsuch a structure in which two gates are electrically connected to eachother can have a higher field-effect mobility and thus have higheron-state current than other transistors. Consequently, a circuit capableof high-speed operation can be obtained. Furthermore, the area occupiedby a circuit portion can be reduced. The use of a transistor having highon-state current can reduce signal delay in wirings and can suppressdisplay unevenness even in an input/output device in which the number ofwirings is increased in accordance with the increase in size orresolution. Moreover, with such a structure, a highly reliabletransistor can be formed.

The transistors 201 a and 203 a may have the same structure or differentstructures. That is, a transistor included in a driver circuit portionand a transistor included in a display portion may have the samestructure or different structures. The driver circuit portion mayinclude transistors having a plurality of structures. The displayportion may include transistors having a plurality of structures. Forexample, a transistor having a structure in which two gates areelectrically connected to each other is preferably used for one or moreof a shift register circuit, a buffer circuit, and a protection circuitincluded in a scan line driver circuit.

The transistors 201 a and 203 a are covered with an insulating film 217and an insulating film 219. Note that the insulating films 217 and 219can be regarded as the components of the transistors 201 a and 203 a.The insulating film 217 preferably has an effect of suppressingdiffusion of impurities into a semiconductor included in a transistor.For example, for the insulating film 217, a material through whichimpurities such as water and hydrogen are hardly diffused is preferablyused. As the insulating film 219, an insulating film having aplanarization function is preferably selected in order to reduce surfaceunevenness due to the transistor.

In the transistor 201 a, the oxide semiconductor film 223 is used as asemiconductor layer, and the oxide conductor film 227 is used as a gate.In that case, it is preferable that the oxide semiconductor film 223 andthe oxide conductor film 227 be formed using an oxide semiconductor.

The resistivity of an oxide semiconductor can be easily controlled in amanufacturing process of the input/output device; thus, an oxidesemiconductor can be favorably used as a material of a semiconductorfilm and a conductive film. When two or more layers included in theinput/output device are formed using oxide semiconductors containing thesame metal element, the same manufacturing apparatus (e.g., depositionapparatus or processing apparatus) can be used in two or more steps andmanufacturing cost can thus be reduced.

An oxide semiconductor is a material that transmits visible light andcan therefore be favorably used for an element that transmits visiblelight.

Forming the oxide semiconductor film 223 and the oxide conductor film227 using the same metal element can reduce the manufacturing cost. Forexample, when metal oxide targets with the same metal composition areused, the manufacturing cost can be reduced and the same etching gas orthe same etchant can be used in processing the oxide semiconductorfilms. Even when the oxide semiconductor film 223 and the oxideconductor film 227 contain the same metal element, they have differentcompositions in some cases. For example, a metal element in a film isreleased during the manufacturing process of the input/output device,which might result in different metal compositions.

The transistors 201 a and 203 a preferably include the oxidesemiconductor film 223 that is highly purified to reduce the formationof oxygen vacancies. Accordingly, the current in an off state (off-statecurrent) of the transistors can be made small. Thus, an electricalsignal such as an image signal can be held for a longer period, and awriting interval can be set longer in an on state. Accordingly,frequency of refresh operation can be reduced, which leads to an effectof reducing power consumption.

In the transistors 201 a and 203 a, relatively high field-effectmobility can be obtained, whereby high-speed operation is possible. Whensuch a transistor that can operate at high speed is used for theinput/output device, a transistor in a display portion and a transistorin a driver circuit portion can be formed over one substrate. That is,since a semiconductor device formed of a silicon wafer or the like isnot additionally needed as a driver circuit, the number of components ofthe input/output device can be reduced. In addition, the transistor thatcan operate at high speed can be used also in the display portion,whereby a high-quality image can be provided.

The liquid crystal element 207 a is a liquid crystal element having afringe field switching (FFS) mode. The liquid crystal element 207 aincludes a conductive film 251, a conductive film 252, and a liquidcrystal 249. Orientation of the liquid crystal 249 can be controlledwith an electric field generated between the conductive films 251 and252. The conductive film 251 can serve as a pixel electrode. Theconductive film 252 can serve as a common electrode.

When a conductive material that transmits visible light is used for theconductive films 251 and 252, the input/output device 300 can serve as atransmissive liquid crystal display device. When a conductive materialthat reflects visible light is used for the conductive film 251 and aconductive material that transmits visible light is used for theconductive film 252, the input/output device 300 can serve as areflective liquid crystal display device.

For example, a material containing one of indium (In), zinc (Zn), andtin (Sn) is preferably used for the conductive material that transmitsvisible light. Specifically, indium oxide, indium tin oxide (ITO),indium zinc oxide, indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium tin oxide to whichsilicon oxide is added, zinc oxide, and zinc oxide to which gallium isadded are given, for example. Note that a film including graphene can beused as well. The film including graphene can be formed, for example, byreducing a film containing graphene oxide.

An oxide conductor film is preferably used as the conductive film 251.Furthermore, an oxide conductor film is preferably used as theconductive film 252. The oxide conductor film preferably contains one ormore kinds of metal elements included in the oxide semiconductor film223. For example, the conductor film 251 preferably contains indium andis further preferably an In-M-Zn oxide (M is Al, Ti, Ga, Ge, Y, Zr, La,Ce, Sn, Mg, Nd, or Hf) film. Similarly, the conductive film 252preferably contains indium and is further preferably the In-M-Zn oxidefilm.

Note that at least one of the conductive films 251 and 252 may be formedusing an oxide semiconductor. As described above, when two or morelayers included in the input/output device are formed using oxidesemiconductors containing the same metal element, the same manufacturingapparatus (e.g., deposition apparatus or processing apparatus) can beused in two or more steps and manufacturing cost can thus be reduced.

For example, when a silicon nitride film containing hydrogen is used asan insulating film 253 and an oxide semiconductor is used for theconductive film 251, the conductivity of the oxide semiconductor can beincreased owing to hydrogen supplied from the insulating film 253.

Examples of a conductive material that reflects visible light includealuminum, silver, and an alloy including any of these metal elements.

The conductive film 251 serving as a pixel electrode is electricallyconnected to a source or a drain of the transistor 203 a. Here, theconductive film 251 is electrically connected to the drain electrode 225b.

The conductive film 252 has a comb-like top surface shape or a topsurface shape provided with a slit (a top surface shape is also referredto as a planar surface shape). The insulating film 253 is providedbetween the conductive films 251 and 252. The conductive film 251 partlyoverlaps with the conductive film 252 with the insulating film 253interposed therebetween. In a region where a coloring film 241 overlapswith the conductive film 251, there is a portion where the conductivefilm 252 is not provided over the conductive film 251.

A conductive film 255 is provided over the insulating film 253. Theconductive film 255 is electrically connected to the conductive film 252and can serve as an auxiliary wiring of the conductive film 252. Withthe auxiliary wiring electrically connected to the common electrode,voltage drop due to the resistance of the common electrode can besuppressed. In that case, a stacked structure of a conductive filmincluding a metal oxide and a conductive film including a metal ispreferably used because these conductive films can be formed by apatterning technique using a half tone mask and thus the process can besimplified.

The conductive film 255 has a lower resistivity than the conductive film252. For example, the conductive film 255 can be formed to have asingle-layer structure or a stacked-layer structure using any of metalmaterials such as molybdenum, titanium, chromium, tantalum, tungsten,aluminum, copper, silver, neodymium, and scandium, and an alloy materialcontaining any of these elements.

To prevent the conductive film 255 from being perceived by the user ofthe input/output device, the conductive film 255 is preferably providedin a position overlapping with a light-blocking film 243 and the like.

The connection portion 205 a is electrically connected to an externalinput terminal through which a signal (e.g., a video signal, a clocksignal, a start signal, and a reset signal) or a potential from theoutside is transmitted to the scan line driver circuit 302. An examplein which the FPC 269 is provided as an external input terminal is shownhere.

The connection portion 205 a includes a conductive film 231 over theinsulating film 213, a conductive film 233 over the conductive film 231,and a conductive film 235 over the conductive film 233. The conductivefilm 231 is electrically connected to the conductive film 235 via theconductive film 233. The conductive film 235 is electrically connectedto the FPC 269 via a connector 267.

The conductive film 231 can be formed using the same material and thesame step as those of the source electrode 225 a and the drain electrode225 b included in the transistors 201 a and 203 a. The conductive film233 can be formed using the same material and the same step as those ofthe conductive film 251 included in the liquid crystal element 207 a.The conductive film 235 can be formed using the same material and thesame step as those of the conductive film 252 included in the liquidcrystal element 207 a. It is preferable to form the conductive filmsincluded in the connection portion 205 a using the same materials andthe same steps as an electrode or a wiring used for a display portion ora driver circuit portion in such a manner because an increase in numberof steps can be prevented.

A substrate 261 is provided with the coloring film 241, thelight-blocking film 243, and an insulating film 245. FIG. 1B illustratesan example in which the substrate 261 has a smaller thickness than thesubstrate 211; however, one embodiment of the present invention is notlimited to this example. One of the substrates 261 and 211 may bethinner than the other, or the substrates 261 and 211 may have the samethickness. It is preferable to make the substrate on the display surfaceside (the side near a sensing target) thin because the detectionsensitivity of a sensor element can be increased.

The coloring film 241 partly overlaps with the liquid crystal element207 a. The light-blocking film 243 partly overlaps with at least one ofthe transistors 201 a and 203 a.

The insulating film 245 preferably has a function of an overcoatpreventing impurities contained in the coloring film 241, thelight-blocking film 243, and the like from diffusing into the liquidcrystal 249. The insulating film 245 is not necessarily provided.

Note that an alignment film in contact with the liquid crystal 249 maybe provided. The alignment film can control the alignment of the liquidcrystal 249. For example, in FIG. 1B, an alignment film may be formed tocover the conductive film 252 or may be formed between the insulatingfilm 245 and the liquid crystal 249. The insulating film 245 may serveas an alignment film and an overcoat.

The input/output device 300 includes a spacer 247. The spacer 247 has afunction of preventing the distance between the substrate 211 and thesubstrate 261 from being shorter than or equal to a certain distance.

FIG. 1B illustrates an example in which the spacer 247 is provided overthe insulating film 253 and the conductive film 252; however, oneembodiment of the present invention is not limited thereto. The spacer247 may be provided on the substrate 211 side or on the substrate 261side. For example, the spacer 247 may be formed on the insulating film245. Moreover, although FIG. 1B illustrates an example in which thespacer 247 is in contact with the insulating films 253 and 245, thespacer 247 is not necessarily in contact with a component provided onthe substrate 211 side or on the substrate 261 side.

A particulate spacer may be used as the spacer 247. Although a materialsuch as silica can be used for the particulate spacer, an elasticmaterial such as a resin or rubber is preferably used. In that case, theparticulate spacer may be vertically crushed.

The substrates 211 and 261 are attached to each other with a bondinglayer 265. A region surrounded by the substrate 211, the substrate 261,and the bonding layer 265 is filled with the liquid crystal 249.

Note that when the input/output device 300 serves as a transmissiveliquid crystal display device, two polarizing plates are provided sothat a display portion is sandwiched between the two polarizing plates.Light from a backlight provided outside the polarizing plate entersthrough the polarizing plate. At this time, the alignment of the liquidcrystal 249 is controlled with a voltage applied between the conductivefilms 251 and 252, whereby optical modulation of light can becontrolled. In other words, the intensity of light emitted through thepolarizing plate can be controlled. Light excluding light in aparticular wavelength range is absorbed by the coloring film 241, sothat red, blue, or green light is emitted.

In addition to the polarizing plate, a circularly polarizing plate canbe used, for example. As the circularly polarizing plate, for example, astack including a linear polarizing plate and a quarter-wave retardationplate can be used. With the circularly polarizing plate, the viewingangle dependence of display of the input/output device can be reduced.

Note that the liquid crystal element 207 a is an element using an FFSmode here; however, one embodiment of the present invention is notlimited thereto, and a liquid crystal element using any of a variety ofmodes can be used. For example, a liquid crystal element using avertical alignment (VA) mode, a twisted nematic (TN) mode, an in-planeswitching (IPS) mode, an axially symmetric aligned micro-cell (ASM)mode, an optically compensated birefringence (OCB) mode, a ferroelectricliquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC)mode, or the like can be used.

Furthermore, a normally black liquid crystal display device, forexample, a transmissive liquid crystal display device using a verticalalignment (VA) mode, may be used as the input/output device 300. As avertical alignment mode, a multi-domain vertical alignment (MVA) mode, apatterned vertical alignment (PVA) mode, or an ASV mode can be employed,for example.

Note that the liquid crystal element is an element that controlstransmission or non-transmission of light by utilizing an opticalmodulation action of liquid crystal. Note that optical modulation actionof a liquid crystal is controlled by an electric field applied to theliquid crystal (including a horizontal electric field, a verticalelectric field, and an oblique electric field). As the liquid crystalused for the liquid crystal element, thermotropic liquid crystal,low-molecular liquid crystal, high-molecular liquid crystal, polymerdispersed liquid crystal (PDLC), ferroelectric liquid crystal,anti-ferroelectric liquid crystal, or the like can be used. Such aliquid crystal material exhibits a cholesteric phase, a smectic phase, acubic phase, a chiral nematic phase, an isotropic phase, or the likedepending on conditions.

As the liquid crystal material, a positive liquid crystal or a negativeliquid crystal may be used, and an appropriate liquid crystal materialcan be used depending on the mode and design to be used.

Alternatively, in the case of employing a horizontal electric fieldmode, a liquid crystal exhibiting a blue phase for which an alignmentfilm is unnecessary may be used. A blue phase is one of liquid crystalphases, which is generated just before a cholesteric phase changes intoan isotropic phase while temperature of cholesteric liquid crystal isincreased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which 5 weight % or more of achiral material is mixed is used for the liquid crystal 249 in order toincrease the temperature range. The liquid crystal composition thatincludes liquid crystal exhibiting a blue phase and a chiral materialhas a short response time and has optical isotropy. In addition, theliquid crystal composition that includes liquid crystal exhibiting ablue phase and a chiral material does not need alignment treatment andhas a small viewing angle dependence. In addition, since an alignmentfilm does not need to be provided and rubbing treatment is unnecessary,electrostatic discharge damage caused by the rubbing treatment can beprevented and defects or damage of the liquid crystal display device inthe manufacturing process can be reduced.

A substrate with which a sensing target, such as a finger or a stylus,is to be in contact may be provided above the substrate 261. In thatcase, a polarizing plate or a circularly polarizing plate is preferablyprovided between the substrate 261 and the above substrate. In thatcase, a protective layer (such as a ceramic coat) is preferably providedover the above substrate. The protective layer can be formed using aninorganic insulating material such as silicon oxide, aluminum oxide,yttrium oxide, or yttria-stabilized zirconia (YSZ). Alternatively,tempered glass may be used for the substrate. Physical or chemicalprocessing by an ion exchange method, a wind tempering method, or thelike is performed on the tempered glass, so that compressive stress isapplied on the surface.

FIG. 2A is a cross-sectional view of two adjacent pixels. Two subpixelsillustrated in FIG. 2A are included in respective pixels.

In the input/output device of FIG. 2A, capacitance formed between theconductive film 252 in the left subpixel and the conductive film 252 inthe right subpixel is utilized to sense proximity, touch, or the like ofa sensing target. That is, in the input/output device of one embodimentof the present invention, the conductive film 252 serves as a commonelectrode of the liquid crystal element and an electrode of the sensorelement.

As described above, an electrode included in the liquid crystal elementalso serves as an electrode included in the sensor element in theinput/output device of one embodiment of the present invention; thus,the manufacturing process can be simplified and the manufacturing costcan be reduced. In addition, the thickness and weight of theinput/output device can be reduced.

The conductive film 252 is electrically connected to the conductive film255 serving as an auxiliary wiring. With the conductive film 255, theresistance of the electrode of the sensor element can be lowered. Withthe lowered resistance of the electrode of the sensor element, the timeconstant of the electrode of the sensor element can be small. Thesmaller the time constant of the electrode of the sensor element is, thehigher the detection sensitivity and the detection accuracy are.

When the capacitance between the electrode of the sensor element and asignal line is too large, the time constant of the electrode of thesensor element becomes too large in some cases. Thus, an insulating filmhaving a planarizing function is preferably provided between theelectrode of the sensor element and the transistors to reduce thecapacitance between the electrode of the sensor element and the signalline. For example, in FIG. 2A, as the insulating film having aplanarizing function, the insulating film 219 is provided. With theinsulating film 219, the capacitance between the conductive film 252 andthe signal line can be small. Accordingly, the time constant of theelectrode of the sensor element can be small. As described above, thesmaller the time constant of the electrode of the sensor element is, thehigher the detection sensitivity and the detection accuracy are.

For example, the time constant of the electrode of the sensor element isgreater than 0 seconds and smaller than or equal to 1×10⁻⁴ seconds,preferably greater than 0 seconds and smaller than or equal to 5×10⁻⁵seconds, more preferably greater than 0 seconds and smaller than orequal to 5×10⁶ seconds, more preferably greater than 0 seconds andsmaller than or equal to 5×10⁻⁷ seconds, more preferably greater than 0seconds and smaller than or equal to 2×10⁻⁷ seconds. In particular, whenthe time constant is smaller than or equal to 1×10⁻⁶ seconds, highdetection sensitivity can be achieved while the influence of noise isreduced.

[Cross-Sectional Structure Example 2 of Input/Output Device]

FIG. 2B is a cross-sectional view of two adjacent pixels that aredifferent from those in FIG. 2A. Two subpixels illustrated in FIG. 2Bare included in respective pixels. FIG. 3A is a cross-sectional view ofthis case taken along dashed-dotted lines A-B and C-D in FIG. 1A.

Structure example 2 illustrated in FIG. 2B and FIG. 3A differs fromStructure example 1 illustrated in FIGS. 1B and 2A in the stacking orderof the conductive film 251, the conductive film 252, the insulating film253, and the conductive film 255. Note that in Structure example 2, theabove description can be referred to for portions similar to Structureexample 1.

Specifically, in Structure example 2, the conductive film 255 is overthe insulating film 219, the conductive film 252 is over the conductivefilm 255, the insulating film 253 is over the conductive film 252, andthe conductive film 251 is over the insulating film 253.

As illustrated in a liquid crystal element 207 b of FIG. 2B, theconductive film 251 which is provided on the upper side and whose topsurface shape is a comb-like shape or has a slit may serve as a pixelelectrode, and the conductive film 252 provided on the lower side mayserve as a common electrode. The conductive film 251 is electricallyconnected to the source or the drain of the transistor 203 a.

In FIG. 2B, capacitance formed between the conductive film 252 in theleft subpixel and the conductive film 252 in the right subpixel isutilized to sense proximity, touch, or the like of a sensing target.That is, in the input/output device of one embodiment of the presentinvention, the conductive film 252 serves as the common electrode of theliquid crystal element and the electrode of the sensor element.

Note that in Structure example 1 (FIGS. 1B and 2A), the conductive film252 serving as the electrode of the sensor element and the commonelectrode is closer to the display surface side (the side near a sensingtarget) than the conductive film 251 serving as the pixel electrode is.Thus, in some cases, the detection sensitivity of Structure example 1 ishigher than that of Structure example 2 in which the conductive film 251is closer to the display surface side than the conductive film 252 is.

Moreover, the connection portion of Structure example 2 also differsfrom that of Structure example 1 because the stacking order of theconductive film 251, the conductive film 252, the insulating film 253,and the conductive film 255 of Structure example 2 is different fromthat of Structure example 1.

A connection portion 205 b illustrated in FIG. 3A includes theconductive film 231 over the insulating film 213, the conductive film233 over the conductive film 231, and the conductive film 235 over theconductive film 233. The conductive film 233 can be formed using thesame material and the same step as those of the conductive film 252included in the liquid crystal element 207 b. The conductive film 235can be formed using the same material and the same step as those of theconductive film 251 included in the liquid crystal element 207 b.

FIGS. 3B and 3C illustrate other structure examples of the transistorincluded in the input/output device of one embodiment of the presentinvention. As illustrated in a transistor having two gates of FIG. 3B,the two gates are not necessarily electrically connected to each other.As illustrated in FIG. 3C, a top-gate transistor may be included in atleast one of the driver circuit portion and the display portion.

FIGS. 3D to 3F illustrate other structure examples of the liquid crystalelement included in the input/output device of one embodiment of thepresent invention. Both of the conductive films 251 and 252 may have acomb-like top surface shape or a top surface shape provided with a slit(a top surface shape is also referred to as a planar surface shape).

For example, when seen from the top, an end portion of a slit of oneconductive film may overlap with an end portion of a slit of the otherconductive film. A cross-sectional view of this case is illustrated inFIG. 3D.

Alternatively, when seen from the top, a portion where the conductivefilms 251 and 252 are not provided may exist. A cross-sectional view ofthis case is illustrated in FIG. 3E.

Further alternatively, when seen from the top, the conductive films 251and 252 may overlap with each other partly. A cross-sectional view ofthis case is illustrated in FIG. 3F.

[Cross-Sectional Structure Example 3 of Input/Output Device]

FIG. 4 is a cross-sectional view taken along dashed-dotted lines A-B andC-D in FIG. 1A that is different from the cross-sectional views of FIG.1B and FIG. 3A.

The structures of the transistors included in the display portion 301and in the scan line driver circuit 302 of Structure example 3 in FIG. 4differ from those of Structure example 1 in FIGS. 1B and 2A. Note thatin Structure example 3, the above description can be referred to forportions similar to Structure example 1.

A transistor 201 b has a structure in which an oxide semiconductor filmwhere a channel is formed is sandwiched between two gates. Thetransistor 201 b differs from the transistor 201 a in that the gateelectrode 221 is directly in contact with the oxide conductor film 227.Like this, two gates may be electrically connected to each other with nolayer interposed therebetween.

Like the transistor 201 b, a transistor 203 b has a structure in whichthe oxide semiconductor film 223 where a channel is formed is sandwichedbetween two gates. Like this, a transistor having two gates may beapplied not only in the driver circuit portion but also in the displayportion. Note that although not illustrated, also in the transistor 203b, the gate electrode 221 is preferably electrically connected to theoxide conductor film 227.

Note that the shorter the distance between the oxide conductor film 227and the electrode of the sensor element is, the more a problem ofchanging the potential of the electrode of the sensor element due to aninfluence of the oxide conductor film 227 is caused. In one embodimentof the present invention, the oxide conductor film 227 and the electrodeof the sensor element are formed in different layers. This is preferablebecause the electrode of the sensor element is not easily affected bythe oxide conductor film 227.

[Cross-Sectional Structure Example 4 of Input/Output Device]

FIG. 5 is a cross-sectional view taken along dashed-dotted lines A-B andC-D in FIG. 1A that is different from the cross-sectional views of FIG.1B, FIG. 3A, and FIG. 4.

Structure example 4 in FIG. 5 differs from Structure example 1 in FIGS.1B and 2A in the structure of the transistor in the scan line drivercircuit 302 and in the substrate provided with the spacer 247. Note thatin Structure example 4, the above description can be referred to forportions similar to Structure example 1.

A transistor 201 c has a structure in which an oxide semiconductor filmwhere a channel is formed is sandwiched between two gates. Thetransistor 201 c differs from the transistor 201 a in the formationposition of the oxide conductor film 227. Specifically, the insulatingfilm 217 is over the insulating film 215, an insulating film 218 havinga planarizing function is over the insulating film 217, and the oxideconductor film 227 is over the insulating film 218. As described above,the oxide conductor film 227 may be formed over an insulating filmhaving a planarizing function. The transistor 201 c is covered with theinsulating film 219 having a planarizing function. Note that FIG. 5illustrates an example in which the oxide conductor film 227 iselectrically connected to the gate electrode 221 via the conductive film226; however, as illustrated in FIG. 4, the oxide conductor film 227 maybe directly in contact with the gate electrode 221.

Furthermore, FIG. 5 illustrates an example in which the spacer 247 isprovided on the insulating film 245. Like this, the spacer 247 may bearranged on the substrate 261 side.

[Cross-Sectional Structure Example 5 of Input/Output Device]

FIG. 6 is a cross-sectional view taken along dashed-dotted lines A-B andC-D in FIG. 1A that is different from the cross-sectional views of FIG.1B, FIG. 3A, FIG. 4, and FIG. 5.

Structure example 5 in FIG. 6 differs from Structure example 1 in FIGS.1B and 2A in the position of the coloring film 241. Note that inStructure example 5, the above description can be referred to forportions similar to Structure example 1.

The coloring film 241 is not necessarily formed on the counter substrate(the substrate 261) side. As illustrated in FIG. 6, the coloring film241 may be formed over the substrate 211 provided with the transistorsand the like. Accordingly, a decrease in yield and display qualitycaused by a decrease in alignment accuracy of the substrates 211 and 261due to an increase in resolution of the display of the input/outputdevice can be suppressed.

[Cross-Sectional Structure Example 6 of Input/Output Device]

FIG. 34 is a cross-sectional view of an input/output device that isdifferent from the input/output devices in the above-described structureexamples. The input/output device of one embodiment of the presentinvention is not limited to a full-in-cell touch panel in which anelectrode and the like included in a sensor element are provided onlyover a substrate supporting a display element. As illustrated in theinput/output device of FIG. 34, an electrode included in a sensorelement may be provided on a counter substrate side.

FIG. 34 illustrates an example in which a conductive film 254 is formedover a surface of the substrate 261 that is opposite to a surface onwhich the coloring film 241 and the like are formed. The conductive film254 is electrically connected to an FPC 259 via a connector 257. In theinput/output device 300 of FIG. 34, capacitance formed between theconductive film 252 and the conductive film 254 is utilized to senseproximity, touch, or the like of a sensing target. That is, in theinput/output device of one embodiment of the present invention, theconductive film 252 serves as the common electrode of the liquid crystalelement and one electrode of the sensor element. In this manner, thecommon electrode of the liquid crystal element may serve as the oneelectrode or a pair of electrodes of the sensor element.

Furthermore, FIG. 34 illustrates an example in which the conductive film255 is formed over the conductive film 252. The electrode of the liquidcrystal element may be over or under a conductive film that can serve asan auxiliary wiring of the electrode.

Next, the details of the materials and the like that can be used forcomponents of the input/output device of this embodiment are described.Note that description on the components already described is omitted insome cases. The materials described below can be used for theinput/output device described in a later embodiment and its components.

<<Substrate>>

There is no particular limitation on a material and the like of thesubstrates included in the input/output device 300 as long as thematerial has heat resistance high enough to withstand at least heattreatment performed later. For example, a glass substrate, a ceramicsubstrate, a quartz substrate, a sapphire substrate, or the like may beused as the substrates. Alternatively, a single crystal semiconductorsubstrate or a polycrystalline semiconductor substrate made of siliconor silicon carbide, a compound semiconductor substrate made of silicongermanium or the like, an SOI substrate, or the like may be used. Stillalternatively, any of these substrates provided with a semiconductorelement may be used as the substrate. Furthermore, any of thesesubstrates further provided with a semiconductor element may be used asthe substrate. In the case where a glass substrate is used as thesubstrate, a glass substrate having any of the following sizes can beused: the 6th generation (1500 mm×1850 mm), the 7th generation (1870mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation(2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, alarge-sized display device can be manufactured. Alternatively, aflexible substrate may be used as the substrate 211, and the transistor,the capacitor, and the like may be formed directly on the flexiblesubstrate.

The weight and thickness of the input/output device can be reduced byusing a thin substrate. Furthermore, a flexible input/output device canbe obtained by using a substrate that is thin enough to haveflexibility.

Other than the above, a transistor can be formed using varioussubstrates as the substrates 211 and 261. The type of a substrate is notlimited to a certain type. Examples of the substrate include a plasticsubstrate, a metal substrate, a stainless steel substrate, a substrateincluding stainless steel foil, a tungsten substrate, a substrateincluding tungsten foil, a flexible substrate, an attachment film, paperincluding a fibrous material, and a base film. As an example of a glasssubstrate, a barium borosilicate glass substrate, an aluminoborosilicateglass substrate, a soda lime glass substrate, or the like can be given.Examples of a flexible substrate include a flexible synthetic resin suchas plastics typified by polyethylene terephthalate (PET), polyethylenenaphthalate (PEN), and polyether sulfone (PES), and acrylic. Examples ofan attachment film are attachment films formed using polypropylene,polyester, polyvinyl fluoride, polyvinyl chloride, and the like.Examples of the material for the base film include polyester, polyamide,polyimide, inorganic vapor deposition film, and paper. Specifically, theuse of semiconductor substrates, single crystal substrates, SOIsubstrates, or the like enables the manufacture of small-sizedtransistors with a small variation in characteristics, size, shape, orthe like and with high current capability. A circuit using suchtransistors achieves lower power consumption of the circuit or higherintegration of the circuit.

Note that a transistor may be formed using one substrate, and then thetransistor may be transferred to another substrate. Examples of asubstrate to which a transistor is transferred include, in addition tothe above substrate over which the transistor can be formed, a papersubstrate, a cellophane substrate, a stone substrate, a wood substrate,a cloth substrate (including a natural fiber (e.g., silk, cotton, orhemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), aregenerated fiber (e.g., acetate, cupra, rayon, or regeneratedpolyester), and the like), a leather substrate, and a rubber substrate.When such a substrate is used, a transistor with excellent properties ora transistor with low power consumption can be formed, a device withhigh durability or high heat resistance can be provided, or reduction inweight or thickness can be achieved.

<<Transistor>>

The structure of the transistors in the input/output device of oneembodiment of the present invention is not particularly limited. Forexample, a planar transistor, a staggered transistor, or an invertedstaggered transistor may be used. A top-gate transistor or a bottom-gatetransistor may be used. Gate electrodes may be provided above and belowa channel. A semiconductor material used for the transistor is notparticularly limited, and for example, an oxide semiconductor, silicon,or germanium can be used.

There is no particular limitation on the crystallinity of asemiconductor material used for the transistor either, and an amorphoussemiconductor or a semiconductor having crystallinity (amicrocrystalline semiconductor, a polycrystalline semiconductor, asingle-crystal semiconductor, or a semiconductor partly includingcrystal regions) may be used. It is preferable that a semiconductorhaving crystallinity be used, in which case deterioration of thetransistor characteristics can be suppressed.

As a semiconductor material for the semiconductor layer of thetransistor, an element of Group 14, a compound semiconductor, or anoxide semiconductor can be used, for example. Typically, a semiconductorcontaining silicon, a semiconductor containing gallium arsenide, anoxide semiconductor containing indium, or the like can be used.

An oxide semiconductor is preferably used as a semiconductor in which achannel of the transistor is formed. In particular, an oxidesemiconductor having a wider band gap than silicon is preferably used. Asemiconductor material having a wider band gap and a lower carrierdensity than silicon is preferably used because the off-state current ofthe transistor can be reduced.

For example, it is preferable that the oxide semiconductor contain atleast indium (In) or zinc (Zn). It is further preferable that the oxidesemiconductor contains an oxide represented by an In-M-Zn oxide (M is ametal such as Al, Ti, Ga, Ge, Y, Zr, La, Ce, Sn, Mg, Nd, or Hf).

As the semiconductor layer, it is particularly preferable to use anoxide semiconductor film including a plurality of crystal parts whosec-axes are aligned substantially perpendicular to a surface on which thesemiconductor layer is formed or the top surface of the semiconductorlayer and having no grain boundary between adjacent crystal parts.

There is no grain boundary in such an oxide semiconductor; therefore,generation of a crack in an oxide semiconductor film that is caused bystress when a display panel is bent is prevented. Therefore, such anoxide semiconductor can be preferably used for a flexible input/outputdevice that is used in a bent state, or the like.

Moreover, the use of such an oxide semiconductor for the semiconductorlayer makes it possible to provide a highly reliable transistor in whicha variation in electrical characteristics is suppressed.

Charge accumulated in a capacitor through a transistor can be held for along time because of the low off-state current of the transistor. Whensuch a transistor is used for a pixel, operation of a driver circuit canbe stopped while a gray scale of an image displayed in each displayregion is maintained. As a result, a display device with an extremelylow power consumption can be obtained.

<<Oxide Semiconductor Film>>

It is preferable that the oxide semiconductor film 223 includes a filmrepresented by an In-M-Zn oxide that contains, for example, at leastindium (In), zinc (Zn), and M (a metal such as Al, Ti, Ga, Ge, Y, Zr,La, Ce, Sn, Mg, Nd, or Hf). In order to reduce variations in electricalcharacteristics of the transistor including the oxide semiconductor, theoxide semiconductor preferably contains a stabilizer in addition to theabove elements.

Examples of the stabilizer, including metals that can be used as M, aregallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al), and zirconium (Zr).Other examples of the stabilizer are lanthanoid such as lanthanum (La),cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium(Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho),erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).

As an oxide semiconductor included in the oxide semiconductor film 223,any of the following oxides can be used, for example: an In—Ga—Zn-basedoxide, an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, anIn—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide,an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-basedoxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, anIn—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide,an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-basedoxide, an In—Lu—Zn-based oxide, an In—Sn—Ga—Zn-based oxide, anIn—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, anIn—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, and anIn—Hf—Al—Zn-based oxide.

Note that here, an “In—Ga—Zn-based oxide” means an oxide containing In,Ga, and Zn as its main components, and there is no limitation on theratio of In:Ga:Zn. The In—Ga—Zn-based oxide may contain another metalelement in addition to In, Ga, and Zn.

Note that in the case where the oxide semiconductor film 223 includes anIn-M-Zn oxide, when the summation of In and M is assumed to be 100atomic %, the atomic proportions of In and M are preferably higher than25 atomic % and lower than 75 atomic %, respectively, more preferablyhigher than 34 atomic % and lower than 66 atomic %, respectively.

The energy gap of the oxide semiconductor film 223 is 2 eV or more,preferably 2.5 eV or more, more preferably 3 eV or more. In this manner,the off-state current of the transistor can be reduced by using an oxidesemiconductor having a wide energy gap.

The thickness of the oxide semiconductor film 223 is greater than orequal to 3 nm and less than or equal to 200 nm, preferably greater thanor equal to 3 nm and less than or equal to 100 nm, more preferablygreater than or equal to 3 nm and less than or equal to 50 nm.

In the case where the oxide semiconductor film 223 includes an In-M-Znoxide (M is Al, Ti, Ga, Ge, Y, Zr, La, Ce, Sn, Mg, Nd, or Hf), it ispreferable that the atomic ratio of metal elements of a sputteringtarget used for forming a film of the In-M-Zn oxide satisfy In≥M andZn≥M. As the atomic ratio of the metal elements of such a sputteringtarget, In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=3:1:2, In:M:Zn=1:3:4,In:M:Zn=1:3:6, and the like are given. Note that the atomic ratio ofmetal elements in the formed oxide semiconductor film 223 varies fromthe above atomic ratio of metal elements of the sputtering target withina range of ±40% as an error.

An oxide semiconductor film with a low carrier density is used as theoxide semiconductor film 223. For example, an oxide semiconductor filmwhose carrier density is lower than or equal to 1×10¹⁷/cm³, preferablylower than or equal to 1×10¹⁵/cm³, more preferably lower than or equalto 1×10¹³/cm³, more preferably lower than or equal to 1×10¹¹/cm³ is usedas the oxide semiconductor film 223.

Note that, without limitation to those described above, a material withan appropriate composition can be used depending on requiredsemiconductor characteristics and electrical characteristics (e.g.,field-effect mobility and threshold voltage) of the transistor.Furthermore, to obtain required semiconductor characteristics of thetransistor, it is preferable that the carrier density, the impurityconcentration, the defect density, the atomic ratio of a metal elementto oxygen, the interatomic distance, the density, and the like of theoxide semiconductor film 223 be set to be appropriate.

When silicon or carbon that is one of elements belonging to Group 14 iscontained in the oxide semiconductor film 223, oxygen vacancies areincreased, and the oxide semiconductor film 223 has n-type conductivity.Thus, the concentration of silicon or carbon (measured by secondary ionmass spectrometry (SIMS)) of the oxide semiconductor film 223 is lowerthan or equal to 2×10¹⁸ atoms/cm³, preferably lower than or equal to2×10¹⁷ atoms/cm³.

Furthermore, the concentration of alkali metal or alkaline earth metalin the oxide semiconductor film 223, which is measured by SIMS, is lowerthan or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generatecarriers when bonded to an oxide semiconductor, in which case theoff-state current of the transistor might be increased. Therefore, it ispreferable to reduce the concentration of alkali metal or alkaline earthmetal in the oxide semiconductor film 223.

When nitrogen is contained in the oxide semiconductor film 223,electrons serving as carriers are generated to increase the carrierdensity, so that the oxide semiconductor film 223 easily has n-typeconductivity. Thus, a transistor including an oxide semiconductor thatcontains nitrogen is likely to be normally on. For this reason, nitrogenin the oxide semiconductor film is preferably reduced as much aspossible; the concentration of nitrogen that is measured by SIMS ispreferably set to, for example, lower than or equal to 5×10¹⁸ atoms/cm³.

The oxide semiconductor film 223 may have a non-single-crystalstructure, for example. The non-single-crystal structure includes ac-axis aligned crystalline oxide semiconductor (CAAC-OS) that isdescribed later, a polycrystalline structure, a microcrystallinestructure that is described later, or an amorphous structure, forexample. Among the non-single-crystal structures, an amorphous structurehas the highest density of defect states, whereas CAAC-OS has the lowestdensity of defect states.

The oxide semiconductor film 223 may have an amorphous structure, forexample. An oxide semiconductor film having an amorphous structure hasdisordered atomic arrangement and no crystalline component, for example.Alternatively, an oxide film having an amorphous structure has, forexample, an absolutely amorphous structure and no crystal part.

Note that the oxide semiconductor film 223 may be a mixed film includingtwo or more of the following: a region having an amorphous structure, aregion having a microcrystalline structure, a region having apolycrystalline structure, a region of CAAC-OS, and a region having asingle-crystal structure. The mixed film has a single-layer structureincluding, for example, two or more of a region having an amorphousstructure, a region having a microcrystalline structure, a region havinga polycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure in some cases. Furthermore, the mixed film hasa stacked-layer structure of two or more of the following in some cases:the region having an amorphous structure, the region having amicrocrystalline structure, the region having a polycrystallinestructure, the region of CAAC-OS, and the region having a single-crystalstructure.

Alternatively, silicon is preferably used as a semiconductor in which achannel of the transistor is formed. Although amorphous silicon may beused as silicon, silicon having crystallinity is particularlypreferable. For example, microcrystalline silicon, polycrystallinesilicon, single crystal silicon, or the like is preferably used. Inparticular, polycrystalline silicon can be formed at a lower temperaturethan single crystal silicon and has a higher field-effect mobility and ahigher reliability than amorphous silicon. When such a polycrystallinesemiconductor is used for a pixel, the aperture ratio of the pixel canbe improved. Even in the case where an extremely high resolutioninput/output device is manufactured, a gate driver circuit and a sourcedriver circuit can be formed over a substrate over which the pixels areformed, and the number of components of an electronic device can bereduced.

<<Method for Controlling Resistivity of Oxide Semiconductor>>

An oxide semiconductor is a semiconductor material whose resistance canbe controlled by oxygen vacancies in the film and/or the concentrationof impurities such as hydrogen or water in the film. Thus, theresistivity of the oxide conductor film can be controlled by selectingtreatment for increasing oxygen vacancies and/or impurity concentrationon the oxide semiconductor film or treatment for reducing oxygenvacancies and/or impurity concentration on the oxide semiconductor film.

Note that such an oxide conductor film formed using an oxidesemiconductor film can be referred to as an oxide semiconductor filmhaving a high carrier density and a low resistance, an oxidesemiconductor film having conductivity, or an oxide semiconductor filmhaving high conductivity.

Specifically, plasma treatment is performed on an oxide semiconductorfilm to be the oxide conductor film 227 serving as a gate to increaseoxygen vacancies and/or impurities such as hydrogen or water in theoxide semiconductor film; accordingly, the oxide semiconductor film canhave a high carrier density and a low resistance. Alternatively, theinsulating film 217 containing hydrogen is formed in contact with theoxide semiconductor film to diffuse hydrogen from the insulating film217 containing hydrogen to the oxide semiconductor film, so that theoxide semiconductor film can have a high carrier density and a lowresistance.

The insulating film 215 is formed over the oxide semiconductor film 223so that the oxide semiconductor film 223 is not subjected to the aboveplasma treatment. Since the insulating film 215 is provided, the oxidesemiconductor film 223 is not in contact with the insulating film 217containing hydrogen. The insulating film 215 can be formed using aninsulating film capable of releasing oxygen, in which case oxygen can besupplied to the oxide semiconductor film 223. The oxide semiconductorfilm 223 to which oxygen is supplied is an oxide semiconductor in whichoxygen vacancies in the film or at the interface are reduced and whichhas a high resistance. Note that as the insulating film capable ofreleasing oxygen, a silicon oxide film or a silicon oxynitride film canbe used, for example.

To obtain an oxide semiconductor film having a low resistivity,hydrogen, boron, phosphorus, or nitrogen may be introduced into theoxide semiconductor film by an ion implantation method, an ion dopingmethod, a plasma immersion ion implantation method, or the like.

As the plasma treatment to be performed on the oxide conductor film 227,plasma treatment using a gas containing one of a rare gas (He, Ne, Ar,Kr, or Xe), phosphorus, boron, hydrogen, and nitrogen is typical.Specifically, plasma treatment in an Ar atmosphere, plasma treatment ina mixed gas atmosphere of Ar and hydrogen, plasma treatment in anammonia atmosphere, plasma treatment in a mixed gas atmosphere of Ar andammonia, plasma treatment in a nitrogen atmosphere, or the like can beemployed.

By the plasma treatment, an oxygen vacancy is formed in a lattice fromwhich oxygen is released (or in a portion from which oxygen is released)in the oxide conductor film 227. This oxygen vacancy can cause carriergeneration. Furthermore, when hydrogen is supplied from an insulatingfilm that is in the vicinity of the oxide conductor film 227,specifically, that is in contact with the lower surface or the uppersurface of the oxide conductor film 227, and hydrogen enters the oxygenvacancy, an electron serving as a carrier might be generated.Accordingly, the oxide conductor film 227 whose oxygen vacancies areincreased by the plasma treatment has a higher carrier density than theoxide semiconductor film 223.

The oxide semiconductor film 223 in which oxygen vacancies are reducedand the hydrogen concentration is reduced can be referred to as a highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor film. The term “substantially intrinsic” refers to thestate where an oxide semiconductor has a carrier density lower than1×10¹⁷/cm³, preferably lower than 1×10¹⁵/cm³, further preferably lowerthan 1×10¹³/cm³. Furthermore, the state in which an impurityconcentration is low and the density of defect states is low (the amountof oxygen vacancies is small) is referred to as “highly purifiedintrinsic” or “substantially highly purified intrinsic”. A highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor has few carrier generation sources, and thus has a lowcarrier density in some cases. Thus, a transistor including the oxidesemiconductor film in which a channel region is formed is likely to havepositive threshold voltage (normally-off characteristics). The highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor film 223 has a low density of defect states andaccordingly can have a low density of trap states.

Furthermore, the highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film 223 has an extremely lowoff-state current; even when an element has a channel width of 1×10⁶ μmand a channel length of 10 μm, the off-state current can be less than orequal to the measurement limit of a semiconductor parameter analyzer,i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drain voltage)between a source electrode and a drain electrode of from 1 V to 10 V.Thus, the transistor whose channel region is formed in the oxidesemiconductor film 223 has a small variation in electricalcharacteristics and high reliability.

For example, an insulating film containing hydrogen, that is, aninsulating film capable of releasing hydrogen, typically, a siliconnitride film, is used as the insulating film 217, whereby hydrogen canbe supplied to the oxide conductor film 227. The insulating film capableof releasing hydrogen preferably has a hydrogen concentration of 1×10²²atoms/cm³ or higher. Such an insulating film is formed in contact withthe oxide conductor film 227, whereby hydrogen can be effectivelycontained in the oxide conductor film 227. In this manner, theabove-described plasma treatment is performed and the structure of theinsulating film in contact with the oxide semiconductor film (or theoxide conductor film) is changed, whereby the resistance of the oxidesemiconductor film (or the oxide conductor film) can be appropriatelyadjusted.

Hydrogen contained in the oxide conductor film 227 reacts with oxygenbonded to a metal atom to be water, and in addition, an oxygen vacancyis formed in a lattice from which oxygen is released (or a portion fromwhich oxygen is released). Due to entry of hydrogen into the oxygenvacancy, an electron serving as a carrier is generated in some cases.Furthermore, in some cases, bonding of part of hydrogen to oxygen bondedto a metal atom causes generation of an electron serving as a carrier.Thus, the oxide conductor film 227 containing hydrogen has a highercarrier density than the oxide semiconductor film 223.

Hydrogen in the oxide semiconductor film 223 of the transistor in whicha channel region is formed is preferably reduced as much as possible.Specifically, in the oxide semiconductor film 223, the concentration ofhydrogen that is measured by SIMS is set to 2×10²⁰ atoms/cm³ or lower,preferably 5×10¹⁹ atoms/cm³ or lower, more preferably 1×10¹⁹ atoms/cm³or lower, more preferably 5×10¹⁸ atoms/cm³ or lower, preferably 1×10¹⁸atoms/cm³ or lower, more preferably 5×10¹⁷ atoms/cm³ or lower, stillmore preferably 1×10¹⁶ atoms/cm³ or lower.

On the other hand, the oxide conductor film 227 serving as a gate is alow-resistance oxide conductor film having a high hydrogen concentrationand/or a large amount of oxygen vacancies as compared to the oxidesemiconductor film 223.

A material that can be used for the oxide semiconductor film 223 and amethod for forming the oxide semiconductor film 223 can be applied tothose for the oxide conductor film 227. Note that the oxidesemiconductor film 223 and the oxide conductor film 227 have alight-transmitting property.

Note that a material that can be used for the oxide conductor film 227and a method for forming the oxide conductor film 227 can be applied tothose for the conductive films 251 and 252.

<<Insulating Film>>

An organic insulating material or an inorganic insulating material canbe used as an insulating material that can be used for the insulatingfilm, the overcoat, the spacer, or the like included in the input/outputdevice. Examples of a resin include an acrylic resin, an epoxy resin, apolyimide resin, a polyamide resin, a polyamide-imide resin, a siloxaneresin, a benzocyclobutene-based resin, and a phenol resin. Examples ofan inorganic insulating film include a silicon oxide film, a siliconoxynitride film, a silicon nitride oxide film, a silicon nitride film,an aluminum oxide film, a hafnium oxide film, an yttrium oxide film, azirconium oxide film, a gallium oxide film, a tantalum oxide film, amagnesium oxide film, a lanthanum oxide film, a cerium oxide film, and aneodymium oxide film.

<<Conductive Film>>

For the conductive film such as the gate, the source, and the drain of atransistor and the wiring, the electrode, and the like of theinput/output device, a single-layer structure or a stacked structureusing any of metals such as aluminum, titanium, chromium, nickel,copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungsten,or an alloy containing any of these metals as its main component can beused. For example, a two-layer structure in which a titanium film isstacked over an aluminum film, a two-layer structure in which a titaniumfilm is stacked over a tungsten film, a two-layer structure in which acopper film is stacked over a molybdenum film, a two-layer structure inwhich a copper film is stacked over an alloy film containing molybdenumand tungsten, a two-layer structure in which a copper film is stackedover a copper-magnesium-aluminum alloy film, a three-layer structure inwhich a titanium film or a titanium nitride film, an aluminum film or acopper film, and a titanium film or a titanium nitride film are stackedin this order, a three-layer structure in which a molybdenum film or amolybdenum nitride film, an aluminum film or a copper film, and amolybdenum film or a molybdenum nitride film are stacked in this order,and the like can be given. For example, in the case where the sourceelectrode 225 a and the drain electrode 225 b have a three-layerstructure, it is preferable that each of the first and third layers be afilm formed of titanium, titanium nitride, molybdenum, tungsten, analloy containing molybdenum and tungsten, an alloy containing molybdenumand zirconium, or molybdenum nitride, and that the second layer be afilm formed of a low-resistance material such as copper, aluminum, gold,silver, or an alloy containing copper and manganese. Alight-transmitting conductive material such as indium tin oxide, indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxide towhich silicon oxide is added may be used.

Note that the conductive film may be formed using the above-describedmethod for controlling the resistivity of an oxide semiconductor.

<<Bonding Layer>>

A curable resin such as a thermosetting resin, a photocurable resin, ora two-liquid mixing type curable resin can be used for the bonding layer265. For example, an acrylic resin, a urethane resin, an epoxy resin, ora resin having a siloxane bond can be used.

<<Connector>>

As the connector, anisotropic conductive films (ACF), anisotropicconductive pastes (ACP), and the like can be used for example.

<<Coloring Film>>

The coloring film is a colored layer that transmits light in a specificwavelength range. Examples of a material that can be used for thecoloring film include a metal material, a resin material, and a resinmaterial containing a pigment or dye.

<<Light-Blocking Film>>

The light-blocking film is provided between adjacent coloring films. Forthe light-blocking film, for example, a black matrix can be formed usinga metal material or a resin material containing pigment or dye. Notethat it is preferable to provide the light-blocking film also in aregion other than the display portion, such as a driver circuit portion,in which case undesired leakage of guided light or the like can beinhibited.

[Operation Example of Input/Output Device]

Next, an operation example and the like of the input/output device ofone embodiment of the present invention are described.

FIG. 7A is an equivalent circuit diagram of a pixel circuit provided inthe display portion of the input/output device of one embodiment of thepresent invention.

Each pixel includes at least a transistor 3503 and a liquid crystalelement 3504. A gate of the transistor 3503 is electrically connected toa wiring 3501. One of a source and a drain of the transistor 3503 iselectrically connected to a wiring 3502.

The pixel circuit includes a plurality of wirings extending in the Xdirection (e.g., a wiring 3510_1 and a wiring 3510_2) and a plurality ofwirings extending in the Y direction (e.g., a wiring 3511_1). They areprovided to intersect with each other, and capacitance is formedtherebetween.

Among the pixels provided in the pixel circuit, electrodes on one sideof the liquid crystal elements of some pixels adjacent to each other areelectrically connected to each other to form one block. The block isclassified into two types: an island-shaped block (e.g., a block 3515_1or a block 3515_2) and a linear block extending in the X direction orthe Y direction (e.g., a block 3516 extending in the Y direction). Notethat only part of the pixel circuit is illustrated in FIG. 7A, andactually, these two kinds of blocks are repeatedly arranged in the Xdirection and the Y direction. An electrode on one side of the liquidcrystal element is, for example, a common electrode. An electrode on theother side of the liquid crystal element is, for example, a pixelelectrode.

The wiring 3510_1 (or 3510_2) extending in the X direction iselectrically connected to the island-shaped block 3515_1 (or the block3515_2). Although not illustrated, the wiring 3510_1 extending in the Xdirection is electrically connected to a plurality of island-shapedblocks 3515_1 that are provided discontinuously along the X directionwith the linear blocks therebetween. Furthermore, the wiring 3511_1extending in the Y direction is electrically connected to the linearblock 3516.

FIG. 7B is an equivalent circuit diagram illustrating the connectionrelation between a plurality of wirings extending in the X direction(the wirings 3510_1 to 3510_6 are collectively called a wiring 3510 insome cases) and a plurality of wirings extending in the Y direction(wirings 3511_1 to 3511_6 are collectively called a wiring 3511 in somecases). A common potential can be input to each of the wirings 3510extending in the X direction and each of the wirings 3511 extending inthe Y direction. A pulse voltage can be input to each of the wirings3510 extending in the X direction from a pulse voltage output circuit.Furthermore, each of the wirings 3511 extending in the Y direction canbe electrically connected to the detection circuit. Note that the wiring3510 and the wiring 3511 can be replaced with each other.

An operation example of the input/output device of one embodiment of thepresent invention is described with reference to FIGS. 8A and 8B.

Here, one frame period is divided into a writing period and a sensingperiod. The writing period is a period in which image data is written toa pixel, and the wirings 3501 (also referred to as gate lines or scanlines) are sequentially selected. The sensing period is a period duringwhich sensing is performed by the sensor element.

FIG. 8A is an equivalent circuit diagram in the writing period. In thewiring period, a common potential is input to both the wiring 3510extending in the X direction and the wiring 3511 extending in the Ydirection.

FIG. 8B is an equivalent circuit diagram in the sensing period. In thesensing period, each of the wirings 3511 extending in the Y direction iselectrically connected to the detection circuit. Furthermore, a pulsevoltage is input to the wirings 3510 extending in the X direction from apulse voltage output circuit.

FIG. 8C illustrates an example of a timing chart of the input and outputwaveforms of a mutual capacitive sensor element.

In FIG. 8C, sensing of a sensing target is performed in all rows andcolumns in one frame period. FIG. 8C shows two cases in the sensingperiod: a case in which a sensing target is not sensed (not touched) anda case in which a sensing target is sensed (touched).

A pulse voltage is supplied to the wirings 3510_1 to 3510_6 from thepulse voltage output circuit. When the pulse voltage is applied to thewirings 3510_1 to 3510_6, an electric field is generated between a pairof electrodes forming a capacitor, and current flows in the capacitor.The electric field generated between the electrodes is changed by beingblocked by the touch of a finger or a stylus. That is, the capacitancevalue of the capacitor is changed by touch or the like. By utilizingthis, proximity or touch of a sensing target can be sensed.

The wirings 3511_1 to 3511_6 are connected to the detection circuit fordetecting the change in current in the wirings 3511_1 to 3511_6 causedby the change in capacitance value of the capacitor. The current valuedetected in the wirings 3511_1 to 3511_6 is not changed when there is noproximity or touch of a sensing target, and is decreased when thecapacitance value is decreased because of the proximity or touch of asensing target. In order to detect a change in current, the total amountof current may be detected. In that case, an integrator circuit or thelike may be used to detect the total amount of current. Alternatively,the peak value of current may be detected. In that case, current may beconverted into voltage, and the peak value of voltage may be detected.

Note that the waveforms of the wirings 3511_1 to 3511_6 in FIG. 8C showvoltage values corresponding to the detected current values. As shown inFIG. 8C, the timing of the display operation is preferably insynchronization with the timing of the sensing operation.

The waveforms of the wirings 3511_1 to 3511_6 change in accordance withpulse voltages applied to the wirings 3510_1 to 3510_6. When there is noproximity or touch of a sensing target, the waveforms of the wirings3511_1 to 3511_6 uniformly change in accordance with changes in thevoltages of the wirings 3510_1 to 3510_6. The current value is decreasedat the point of proximity or touch of a sensing target and accordinglythe waveform of the voltage value also changes.

By detecting a change in capacitance in this manner, proximity or touchof a sensing target can be sensed. Even when a sensing target such as afinger or a stylus does not touch but only approaches the input/outputdevice, a signal may be detected in some cases.

Note that FIG. 8C shows an example in which a common potential suppliedin the writing period is equal to a low potential supplied in thesensing period in the wiring 3510; however, one embodiment of thepresent invention is not limited thereto. The common potential may bedifferent from the low potential.

It is preferable that, as an example, the pulse voltage output circuitand the detection circuit be formed in one IC. For example, the IC ispreferably mounted on input/output device or a substrate in a housing ofan electronic device. In the case where the input/output device hasflexibility, parasitic capacitance might be increased in a bent portionof the input/output device, and the influence of noise might beincreased. In view of this, it is preferable to use an IC to which adriving method less influenced by noise is applied. For example, it ispreferable to use an IC to which a driving method capable of increasinga signal-noise ratio (S/N ratio) is applied.

It is preferable that a period in which an image is written and a periodin which sensing is performed by a sensor element be separately providedas described above. Thus, a decrease in sensitivity of the sensorelement caused by noise generated when data is written to a pixel can besuppressed.

In one embodiment of the present invention, as shown in FIG. 8D, oneframe period includes one writing period and one sensing period.Alternatively, as shown in FIG. 8E, two sensing periods may be includedin one frame period. When a plurality of sensing periods are included inone frame period, the detection sensitivity can be further increased.For example, two to four sensing periods may be included in one frameperiod.

[Top Structure Example of Sensor Element]

Next, top structure examples of the sensor element of the input/outputdevice of one embodiment of the present invention are described withreference to FIGS. 9A and 9B, FIG. 10, and FIGS. 11A and 11B.

FIG. 9A illustrates a top view of the sensor element. The sensor elementincludes a conductive film 56 a and a conductive film 56 b. Theconductive film 56 a serves as one electrode of the sensor element. Theconductive film 56 b serves as the other electrode of the sensorelement. The sensor element can sense proximity, touch, or the like of asensing target by utilizing capacitance formed between the conductivefilms 56 a and 56 b. Although not illustrated, the conductive films 56 aand 56 b have a comb-like top surface shape or a top surface shapeprovided with a slit in some cases.

In one embodiment of the present invention, the conductive films 56 aand 56 b also serve as the common electrode of the liquid crystalelement.

A plurality of conductive films 56 a are provided in the Y direction andextend in the X direction. A plurality of conductive films 56 b providedin the Y direction are electrically connected to each other via aconductive film 58 extending in the Y direction. FIG. 9A illustrates anexample in which m conductive films 56 a and n conductive films 58 areprovided.

Note that the plurality of conductive films 56 a may be provided in theX direction and in that case, may extend in the Y direction. Theplurality of conductive films 56 b provided in the X direction may beelectrically connected to each other via the conductive film 58extending in the X direction.

As illustrated in FIG. 9B, a conductive film 56 serving as an electrodeof the sensor element is provided over a plurality of pixels 60. Theconductive film 56 corresponds to each of the conductive films 56 a and56 b in FIG. 9A. The pixel 60 is formed of a plurality of subpixelsexhibiting different colors. FIG. 9B shows an example in which the pixel60 is formed of three subpixels, subpixels 60 a, 60 b, and 60 c.

A pair of electrodes of the sensor element is preferably electricallyconnected to respective auxiliary wirings. FIG. 10 illustrates anexample in which the conductive films 56 a and 56 b are electricallyconnected to auxiliary wirings 57 a and 57 b, respectively. Note thatFIG. 10 illustrates an example in which the auxiliary wirings arestacked over the conductive films; however, the conductive films may bestacked over the auxiliary wirings.

The resistivity of the conductive film that transmits visible light isrelatively high in some cases. Thus, the resistance of the pair ofelectrodes of the sensor element is preferably lowered by electricallyconnecting the pair of electrodes of the sensor element to the auxiliarywiring.

When the resistance of the pair of electrodes of the sensor element islowered, the time constant of the pair of electrodes can be small.Accordingly, the detection sensitivity of the sensor element can beincreased; furthermore, the detection accuracy of the sensor element canbe increased.

In the writing period, as illustrated in FIG. 11A, a common potentialVCOM is input to the conductive films 56 a extending in the X directionand the conductive films 58 extending in the Y direction (and theconductive films 56 b electrically connected to the conductive film 58).In contrast, in the sensing period, as illustrated in FIG. 11B, each ofthe conductive films 58 extending in the Y direction (and each of theconductive films 56 b electrically connected to the conductive films 58)is electrically connected to a detection circuit. Furthermore, theconductive films 56 a extending in the X direction are electricallyconnected to pulse voltage output circuits and supplied with a pulsevoltage.

[Top Structure Example of Pixel]

Next, top structure examples of the pixel of the input/output device ofone embodiment of the present invention are described with reference toFIG. 12, FIG. 13, and FIGS. 14A and 14B.

FIG. 12 is a top view of the pixel. FIG. 13 is a view that differs fromFIG. 12 in that the conductive film 252 is indicated by a dotted line.Note that Cross-sectional structure example 1 (FIG. 1A and FIG. 2A) canalso be referred to for the stacking order of the layers.

A plurality of the conductive films 251 each have an island-shaped topsurface shape and are arranged in a matrix. The conductive film 251 iselectrically connected to the source or the drain of the transistor 203a.

The conductive film 252 is provided to overlap with a plurality of theconductive films 251. The conductive film 252 is provided with a slit.Furthermore, the conductive film 252 has an opening in a positionoverlapping with the transistor 203 a.

Here, the conductive film 251 serves as the pixel electrode of theliquid crystal element. The conductive film 252 serves as the commonelectrode of the liquid crystal element. Note that FIG. 12 and FIG. 13illustrate examples in which the conductive film 252 on the upper sideis a common electrode and the conductive film 251 on the lower side is apixel electrode; however, a conductive film on the upper side may be apixel electrode and a conductive film on the lower side may be a commonelectrode.

The conductive film 252 serves as an electrode of the sensor element.

A conductive film 275 is electrically connected to the conductive film255 in a region 277 indicated by a dashed line. The conductive film 255serves as an auxiliary wiring of the conductive film 252 and iselectrically connected to the conductive film 252. The conductive film275 can be formed using the same material and the same step as those ofthe source and the drain of the transistor 203 a.

A plurality of conductive films 252 provided in the Y directioncorresponds to the conductive film 56 b in FIG. 9A and the like. Theconductive film 275 extending in the Y direction corresponds to theconductive film 58 in FIG. 9A and the like. The plurality of conductivefilms 252 provided in the Y direction are electrically connected to theconductive film 275 via the conductive film 255 extending in the Ydirection. At this time, when an oxide conductor film is used as theconductive film 252, it is preferable to connect the conductive film 255formed using a metal, an alloy, or the like to the conductive film 275and electrically connect the conductive film 252 to the conductive film275 via the conductive film 255, in which case the contact resistancecan be lower than that in the case where the conductive film 252 isdirectly in contact with the conductive film 275.

Although FIG. 12 and FIG. 13 illustrate examples in which a pixel 273includes three subpixels, one embodiment of the present invention is notlimited thereto.

FIGS. 14A and 14B illustrate examples of electrodes of a liquid crystalelement.

A pixel electrode and a common electrode included in a liquid crystalelement 207 do not necessarily have a flat-plate like shape and may havea variety of opening patterns (also referred to as a slit) or acomb-like shape including a bending portion or a branching portion.

The liquid crystal element 207 in FIGS. 14A and 14B includes theconductive film 251 that can serve as a pixel electrode and theconductive film 252 that can serve as a common electrode.

The transistor 203 in FIGS. 14A and 14B includes the gate electrode 221,the oxide semiconductor film 223, the source electrode 225 a, and thedrain electrode 225 b. The conductive film 251 is electrically connectedto the drain electrode 225 b.

FIG. 14A illustrates an example in which the conductive film 251 has aslit. FIG. 14B illustrates an example in which the conductive film 251has a comb-like shape. Note that FIGS. 14A and 14B illustrate examplesin which the conductive film 251 is over the conductive film 252;however, the conductive film 252 may be over the conductive film 251.

[Touch Panel Module]

Next, a touch panel module including the input/output device of oneembodiment of the present invention and an IC are described withreference to FIG. 15 and FIGS. 16A to 16C.

FIG. 15 shows a block diagram of a touch panel module 6500. The touchpanel module 6500 includes a touch panel 6510 and an IC 6520. Theinput/output device of one embodiment of the present invention can beapplied to the touch panel 6510.

The touch panel 6510 includes a display portion 6511, an input portion6512, and a scan line driver circuit 6513. The display portion 6511includes a plurality of pixels, a plurality of signal lines, and aplurality of scan lines, and has a function of displaying an image. Theinput portion 6512 serves as a touch sensor by including a plurality ofsensor elements that can sense touch or proximity of a sensing target tothe touch panel 6510. A scan line driver circuit 6513 has a function ofoutputting a scan signal to the scan lines included in the displayportion 6511.

Here, the display portion 6511 and the input portion 6512 are separatelyillustrated as the components of the touch panel 6510 for simplicity;however, what is called an in-cell touch panel having a function ofdisplaying an image and serving as a touch sensor is preferable. Theinput/output device of one embodiment of the present invention is anin-cell touch panel and is thus favorable.

The resolution of the display portion 6511 is preferably as high as HD(number of pixels: 1280×720), FHD (number of pixels: 1920×1080), WQHD(number of pixels: 2560×1440), WQXGA (number of pixels: 2560×1600), 4K(number of pixels: 3840×2160), or 8K (number of pixels: 7680×4320). Inparticular, resolution of 4K, 8K, or higher is preferable. The pixeldensity (definition) of the pixels in the display portion 6511 is higherthan or equal to 300 ppi, preferably higher than or equal to 500 ppi,more preferably higher than or equal to 800 ppi, more preferably higherthan or equal to 1000 ppi, more preferably higher than or equal to 1200ppi. The display portion 6511 with such high resolution and highdefinition enables an increase in a realistic sensation, sense of depth,and the like in personal use such as portable use and home use.

The IC 6520 includes a circuit unit 6501, a signal line driver circuit6502, a sensor driver circuit 6503, and a detection circuit 6504. Thecircuit unit 6501 includes a timing controller 6505, an image processingcircuit 6506, or the like.

The signal line driver circuit 6502 has a function of outputting a videosignal that is an analog signal to a signal line included in the displayportion 6511. For example, the signal line driver circuit 6502 caninclude a shift register circuit and a buffer circuit in combination.The touch panel 6510 may include a demultiplexer circuit connected to asignal line.

The sensor driver circuit 6503 has a function of outputting a signal fordriving a sensor element included in the input portion 6512. As thesensor driver circuit 6503, a shift register circuit and a buffercircuit can be used in combination, for example.

The detection circuit 6504 has a function of outputting, to the circuitunit 6501, an output signal from the sensor element included in theinput portion 6512. The detection circuit 6504 can include an amplifiercircuit and an analog-digital converter (ADC), for example. In thatcase, the detection circuit 6504 converts an analog signal output fromthe input portion 6512 into a digital signal to be output to the circuitunit 6501.

The image processing circuit 6506 included in the circuit unit 6501 hasa function of generating and outputting a signal for driving the displayportion 6511 of the touch panel 6510, a function of generating andoutputting a signal for driving the input portion 6512, and a functionof analyzing a signal output from the input portion 6512 and outputtingthe signal to a CPU 6540.

As specific examples, the image processing circuit 6506 has thefollowing functions: a function of generating a video signal inaccordance with an instruction from the CPU 6540; a function ofperforming signal processing on a video signal in accordance with thespecification of the display portion 6511, converting the signal into ananalog video signal, and supplying the converted signal to the signalline driver circuit 6502; a function of generating a driving signaloutput to the sensor driver circuit 6503 in accordance with aninstruction from the CPU 6540; and a function of analyzing a signalinput from the detection circuit 6504 and outputting the analyzed signalto the CPU 6540 as positional information.

The timing controller 6505 may have a function of generating a signal(e.g., a clock signal or a start pulse signal) on the basis of asynchronization signal included in a video signal or the like on whichthe image processing circuit 6506 performs processing and outputting thesignal to the scan line driver circuit 6513 and the sensor drivercircuit 6503. Furthermore, the timing controller 6505 may have afunction of generating and outputting a signal for determining timingwhen the detection circuit 6504 outputs a signal. Here, the timingcontroller 6505 preferably outputs a signal synchronized with the signaloutput to the scan line driver circuit 6513 and a signal synchronizedwith the signal output to the sensor driver circuit 6503. In particular,it is preferable that a period in which data in a pixel in the displayportion 6511 is rewritten and a period in which sensing is performedwith the input portion 6512 be separately provided. For example, thetouch panel 6510 can be driven by dividing one frame period into aperiod in which data in a pixel is rewritten and a period in whichsensing is performed. Furthermore, detection sensitivity and detectionaccuracy can be increased, for example, by providing two or more sensingperiods in one frame period.

The image processing circuit 6506 can include a processor, for example.A microprocessor such as a digital signal processor (DSP) or a graphicsprocessing unit (GPU) can be used, for example. Furthermore, such amicroprocessor may be obtained with a programmable logic device (PLD)such as a field programmable gate array (FPGA) or a field programmableanalog array (FPAA). The image processing circuit 6506 interprets andexecutes instructions from various programs with the processor toprocess various kinds of data and control programs. The programsexecuted by the processor may be stored in a memory region included inthe processor or a memory device that is additionally provided.

A transistor that includes an oxide semiconductor in a channel formationregion and has an extremely low off-state current can be used in thedisplay portion 6511 or the scan line driver circuit 6513 included inthe touch panel 6510, the circuit unit 6501, the signal line drivercircuit 6502, the sensor driver circuit 6503, or the detection circuit6504 included in the IC 6520, the CPU 6540 provided outside, or thelike. With the use of the transistor having an extremely low off-statecurrent as a switch for holding electric charge (data) that flows into acapacitor serving as a memory element, a long data retention period canbe ensured. For example, by utilizing the characteristic for at leastone of a register and a cache memory of the image processing circuit6506, normally off computing is achieved where the image processingcircuit 6506 operates only when needed and data on the previousprocessing is stored in the memory element in the rest of time; thus,power consumption of the touch panel module 6500 and an electronicdevice on which the touch panel module 6500 is mounted can be reduced.

Although the structure where the circuit unit 6501 includes the timingcontroller 6505 and the image processing circuit 6506 is used here, theimage processing circuit 6506 itself or a circuit having a function ofpart of the image processing circuit 6506 may be provided outside.Alternatively, the CPU 6540 may have a function of the image processingcircuit 6506 or part thereof: For example, the circuit unit 6501 caninclude the signal line driver circuit 6502, the sensor driver circuit6503, the detection circuit 6504, and the timing controller 6505.

Although the example where the IC 6520 includes the circuit unit 6501 isshown here, the structure where the circuit unit 6501 is not included inthe IC 6520 may be employed. In that case, the IC 6520 can include thesignal line driver circuit 6502, the sensor driver circuit 6503, and thedetection circuit 6504. For example, in the case where the touch panelmodule 6500 includes a plurality of ICs, the circuit unit 6501 may beprovided outside the touch panel module 6500 and a plurality of ICs 6520without the circuit unit 6501 may be provided, and alternatively, the IC6520 and an IC including only the signal line driver circuit 6502 can beprovided in combination.

When an IC has a function of driving the display portion 6511 of thetouch panel 6510 and a function of driving the input portion 6512 asdescribed above, the number of ICs mounted on the touch panel module6500 can be reduced; accordingly, cost can be reduced.

FIGS. 16A to 16C each are a schematic diagram of the touch panel module6500 on which the IC 6520 is mounted.

In FIG. 16A, the touch panel module 6500 includes a substrate 6531, acounter substrate 6532, a plurality of FPCs 6533, the IC 6520, ICs 6530,and the like. The display portion 6511, the input portion 6512, and thescan line driver circuits 6513 are provided between the substrate 6531and the counter substrate 6532. The IC 6520 and the ICs 6530 are mountedon the substrate 6531 by a COG method.

The IC 6530 is an IC in which only the signal line driver circuit 6502is provided in the above-described IC 6520 or an IC in which the signalline driver circuit 6502 and the circuit unit 6501 are provided in theabove-described IC 6520. The ICs 6520 and 6530 are supplied with asignal from the outside through the FPCs 6533. Furthermore, a signal canbe output to the outside from at least one of the ICs 6520 and 6530through the FPC 6533.

FIG. 16A illustrates an example where the display portion 6511 ispositioned between two scan line driver circuits 6513. The ICs 6530 areprovided in addition to the IC 6520. Such a structure is preferable inthe case where the display portion 6511 has extremely high resolution.

FIG. 16B illustrates an example where one IC 6520 and one FPC 6533 areprovided. It is preferable to bring functions into one IC 6520 in thismanner because the number of components can be reduced. In the examplein FIG. 16B, the scan line driver circuit 6513 is provided along a sideclose to the FPC 6533 among two short sides of the display portion 6511.

FIG. 16C illustrates an example where a printed circuit board (PCB) 6534on which the image processing circuit 6506 and the like are mounted isprovided. The ICs 6520 and 6530 over the substrate 6531 are electricallyconnected to the PCB 6534 through the FPCs 6533. Here, theabove-described structure without the image processing circuit 6506 canbe applied to the IC 6520.

In each of FIGS. 16A to 16C, the ICs 6520 and 6530 may be mounted on theFPC 6533, not on the substrate 6531. For example, the ICs 6520 and 6530can be mounted on the FPC 6533 by a COF method, a tape automated bonding(TAB) method, or the like.

A structure where the FPC 6533, the IC 6520 (and the IC 6530), or thelike is provided on a short side of the display portion 6511 asillustrated in FIGS. 16A and 16B enables the frame of the display deviceto be narrowed; thus, the structure is preferably used for electronicdevices such as smartphones, mobile phones, and tablet terminals, forexample. The structure with the PCB 6534 illustrated in FIG. 16C can bepreferably used for television devices, monitors, tablet terminals, ornotebook personal computers, for example.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 2

In this embodiment, a manufacturing method of an input/output device ofone embodiment of the present invention is described with reference toFIGS. 17A to 17D, FIGS. 18A to 18C, FIGS. 19A to 19C, FIG. 20, and FIG.21. In this embodiment, a manufacturing method of a transistor is mainlydescribed. Note that the description in Embodiment 1 can be referred tofor the material of each layer.

First, the gate electrode 221 is formed over the substrate 211. Afterthat, the insulating film 213 including insulating films 106 and 107 isformed over the substrate 211 and the gate electrode 221 (see FIG. 17A).

In this embodiment, a glass substrate is used as the substrate 211; atungsten film is used as the gate electrode 221; a silicon nitride filmcapable of releasing hydrogen is used as the insulating film 106; and asilicon oxide film capable of releasing oxygen is used as the insulatingfilm 107.

The insulating film 106 serves as a blocking film that inhibitspenetration of oxygen. For example, in the case where excess oxygen issupplied to at least one of the insulating film 107, the insulating film215, the insulating film 217, and the oxide semiconductor film 223, theinsulating film 106 can inhibit penetration of oxygen.

Note that the insulating film 107 that is in contact with the oxidesemiconductor film 223 serving as a channel region of the transistor ispreferably an oxide insulating film and preferably includes a regionincluding oxygen in excess of the stoichiometric composition(oxygen-excess region). In other words, the insulating film 107 is aninsulating film that is capable of releasing oxygen. In order to providethe oxygen-excess region in the insulating film 107, the insulating film107 can be formed in an oxygen atmosphere, for example. Alternatively,the oxygen excess region may be formed by introduction of oxygen intothe insulating film 107 after the deposition. As a method forintroducing oxygen, an ion implantation method, an ion doping method, aplasma immersion ion implantation method, plasma treatment, or the likemay be employed.

In the case where hafnium oxide is used for one or both of theinsulating films 106 and 107, the following effect is attained. Hafniumoxide has a higher dielectric constant than silicon oxide and siliconoxynitride. Therefore, by using hafnium oxide, the thickness of one orboth of the insulating films 106 and 107 can be made large as comparedwith the case where silicon oxide is used; thus, leakage current due totunnel current can be low. That is, it is possible to provide atransistor with a low off-state current. Moreover, hafnium oxide with acrystalline structure has higher dielectric constant than hafnium oxidewith an amorphous structure. Therefore, it is preferable to use hafniumoxide with a crystalline structure in order to provide a transistor witha low off-state current. Examples of the crystalline structure include amonoclinic crystal structure and a cubic crystal structure. Note thatone embodiment of the present invention is not limited to the aboveexamples.

In this embodiment, a silicon nitride film is formed as the insulatingfilm 106, and a silicon oxide film is formed as the insulating film 107.A silicon nitride film has a higher dielectric constant than a siliconoxide film and needs a larger thickness for a capacitance equivalent tothat of the silicon oxide film. When a silicon nitride film is includedas the insulating film 213 serving as a gate insulating film of thetransistor, the physical thickness of the insulating film can beincreased. This makes it possible to reduce a decrease in withstandvoltage of the transistor and furthermore increase the withstandvoltage, thereby reducing electrostatic discharge damage to thetransistor.

To form the gate electrode 221, a conductive film is formed over thesubstrate 211, the conductive film is patterned so that a desired regionthereof remains, and unnecessary regions are etched.

Next, the oxide semiconductor film 223 is formed in a region overlappingwith the gate electrode 221 over the insulating film 213 (FIG. 17B).

In this embodiment, as the oxide semiconductor film 223, an In—Ga—Znoxide film, which is formed using a metal oxide target withIn:Ga:Zn=1:1:1.2 [atomic ratio], is used.

The oxide semiconductor film 223 can be formed in such a manner that anoxide semiconductor film is formed over the insulating film 213, theoxide semiconductor film is patterned so that a desired region thereofremains, and then unnecessary regions are etched.

After formation of the oxide semiconductor film 223, heat treatment ispreferably performed. The heat treatment is preferably performed at atemperature of higher than or equal to 250° C. and lower than or equalto 650° C., preferably higher than or equal to 300° C. and lower than orequal to 500° C., more preferably higher than or equal to 350° C. andlower than or equal to 450° C., in an inert gas atmosphere, anatmosphere containing an oxidizing gas at 10 ppm or more, or a reducedpressure atmosphere. Alternatively, the heat treatment may be performedfirst in an inert gas atmosphere, and then another heat treatment isperformed in an atmosphere containing an oxidizing gas at 10 ppm or morein order to compensate oxygen released from the oxide semiconductor film223. By this heat treatment, impurities such as hydrogen and water canbe removed from at least one of the insulating film 106, the insulatingfilm 107, and the oxide semiconductor film 223. Note that theabove-described heat treatment may be performed before the oxidesemiconductor film 223 is processed into an island shape.

Note that stable electrical characteristics can be effectively impartedto the transistor in which the oxide semiconductor film 223 serves as achannel region by reducing the concentration of impurities in the oxidesemiconductor film 223 to make the oxide semiconductor film 223intrinsic or substantially intrinsic.

Next, a conductive film is formed over the insulating film 213 and theoxide semiconductor film 223 and is patterned so that a desired regionthereof remains and unnecessary regions are etched, whereby the sourceelectrode 225 a and the drain electrode 225 b are formed over theinsulating film 213 and the oxide semiconductor film 223 (see FIG. 17C).

In this embodiment, a three-layered structure including a tungsten film,an aluminum film, and a titanium film can be used for the sourceelectrode 225 a and the drain electrode 225 b.

After the source electrode 225 a and the drain electrode 225 b areformed, a surface of the oxide semiconductor film 223 may be cleaned.The cleaning may be performed, for example, using a chemical solutionsuch as phosphoric acid. The cleaning using a chemical solution such asa phosphoric acid can remove impurities (e.g., elements contained in thesource electrode 225 a and the drain electrode 225 b) attached to thesurface of the oxide semiconductor film 223. Note that the cleaning isnot necessarily performed, and thus the cleaning may be unnecessary.

In addition, in the step of forming the source electrode 225 a and thedrain electrode 225 b and/or the cleaning step, the thickness of aregion of the oxide semiconductor film 223 that is not covered by thesource electrode 225 a and the drain electrode 225 b might be reduced.

Next, the insulating film 215 including insulating films 114 and 116 isformed over the insulating film 213, the oxide semiconductor film 223,the source electrode 225 a, and the drain electrode 225 b. Then, theinsulating film 215 is patterned so that a desired region thereofremains and unnecessary regions are etched, whereby an opening 141 isformed (see FIG. 17D).

Note that after the insulating film 114 is formed, the insulating film116 is preferably formed in succession without exposure to the air.After the insulating film 114 is formed, the insulating film 116 isformed in succession by adjusting at least one of the flow rate of asource gas, pressure, a high-frequency power, and a substratetemperature without exposure to the air, whereby the concentration ofimpurities attributed to the atmospheric component at the interfacebetween the insulating film 114 and the insulating film 116 can bereduced, and oxygen in the insulating films 114 and 116 can be moved tothe oxide semiconductor film 223; accordingly, the number of oxygenvacancies in the oxide semiconductor film 223 can be reduced.

Note that the insulating film 114 serves as a protective film for theoxide semiconductor film 223 in the step of forming the insulating film116. Consequently, the insulating film 116 can be formed using thehigh-frequency power having a high power density while damage to theoxide semiconductor film 223 is reduced.

In this embodiment, a silicon oxynitride film capable of releasingoxygen is used as the insulating films 114 and 116.

Note that the insulating film 114 that is in contact with the oxidesemiconductor film 223 serving as a channel region of the transistor ispreferably an oxide insulating film capable of releasing oxygen. Inother words, the insulating film capable of releasing oxygen is aninsulating film that includes a region containing oxygen in excess ofthat in the stoichiometric composition (oxygen-excess region). In orderto provide the oxygen-excess region in the insulating film 114, theinsulating film 114 can be formed in an oxygen atmosphere, for example.Alternatively, the oxygen-excess region may be formed by supplyingoxygen to the formed insulating film 114. As a method for supplyingoxygen, an ion implantation method, an ion doping method, a plasmaimmersion ion implantation method, plasma treatment, or the like can beemployed.

The use of the insulating film capable of releasing oxygen as theinsulating film 114 can reduce the number of oxygen vacancies in theoxide semiconductor film 223 by transferring oxygen to the oxidesemiconductor film 223 serving as the channel region of the transistor.For example, the number of oxygen vacancies in the oxide semiconductorfilm 223 can be reduced by using an insulating film having the followingfeature: the number of oxygen molecules released from the insulatingfilm by heat treatment at a temperature higher than or equal to 100° C.and lower than or equal to 700° C., or higher than or equal to 100° C.and lower than or equal to 500° C. is greater than or equal to 1.0×10¹⁸molecules/cm³ when measured by thermal desorption spectroscopy (TDS)analysis.

It is preferable that the number of defects in the insulating film 114be small, typically the spin density corresponding to a signal thatappears at g=2.001 due to a dangling bond of silicon be lower than orequal to 3×10¹⁷ spins/cm³ by ESR measurement. This is because if thedensity of defects in the insulating film 114 is high, oxygen is bondedto the defects and the amount of oxygen that permeates the insulatingfilm 114 is decreased. Furthermore, it is preferable that the amount ofdefects at the interface between the insulating film 114 and the oxidesemiconductor film 223 be small and typically, the spin density of asignal that appears at g=1.89 or more and 1.96 or less due to the defectin the oxide semiconductor film 223 be lower than or equal to 1×10¹⁷spins/cm³, more preferably lower than or equal to the lower limit ofdetection by ESR measurement.

Note that all oxygen entering the insulating film 114 from the outsidemoves to the outside of the insulating film 114 in some cases.Alternatively, some oxygen entering the insulating film 114 from theoutside remains in the insulating film 114 in some cases. Furthermore,movement of oxygen occurs in the insulating film 114 in some cases insuch a manner that oxygen enters the insulating film 114 from theoutside and oxygen contained in the insulating film 114 moves to theoutside of the insulating film 114. When an oxide insulating film thatis permeable to oxygen is formed as the insulating film 114, oxygenreleased from the insulating film 116 provided over the insulating film114 can be moved to the oxide semiconductor film 223 through theinsulating film 114.

The insulating film 114 can be formed using an oxide insulating filmhaving a low density of states due to nitrogen oxide. Note that thedensity of states due to nitrogen oxide can be formed between the energyof the valence band maximum (E_(v) _(_) _(os)) and the energy of theconduction band minimum (E_(c) _(_) _(os)) of the oxide semiconductorfilm. A silicon oxynitride film that releases less nitrogen oxide, analuminum oxynitride film that releases less nitrogen oxide, or the likecan be used as the oxide insulating film.

Note that a silicon oxynitride film that releases a small amount ofnitrogen oxide is a film of which the amount of released ammonia islarger than the amount of released nitrogen oxide in TDS analysis; theamount of released ammonia is typically greater than or equal to 1×10¹⁸molecules/cm³ and less than or equal to 5×10¹⁹ molecules/cm³. The amountof released ammonia corresponds to the released amount caused by heattreatment at a film surface temperature higher than or equal to 50° C.and lower than or equal to 650° C., preferably higher than or equal to50° C. and lower than or equal to 550° C.

Nitrogen oxide (NO_(x); x is greater than 0 and less than or equal to 2,preferably greater than or equal to 1 and less than or equal to 2),typically NO₂ or NO, forms levels in the insulating film 114, forexample. The levels are positioned in the energy gap of the oxidesemiconductor film 223. Therefore, when nitrogen oxide is diffused tothe interface between the insulating film 114 and the oxidesemiconductor film 223, an electron is trapped by the level on theinsulating film 114 side. As a result, the trapped electron remains inthe vicinity of the interface between the insulating film 114 and theoxide semiconductor film 223; thus, the threshold voltage of thetransistor is shifted in the positive direction.

Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Sincenitrogen oxide contained in the insulating film 114 reacts with ammoniacontained in the insulating film 116 in heat treatment, nitrogen oxidecontained in the insulating film 114 is reduced. Therefore, an electronis hardly trapped at the interface between the insulating film 114 andthe oxide semiconductor film 223.

In a transistor using the oxide insulating film as the insulating film114, the shift in threshold voltage can be reduced, which leads to asmaller change in electrical characteristics of the transistor.

Note that in an ESR spectrum obtained at 100 K or lower of theinsulating film 114, by heat treatment in a manufacturing process of thetransistor, typically heat treatment at a temperature lower than 400° C.or lower than 375° C. (preferably higher than or equal to 340° C. andlower than or equal to 360° C.), a first signal that appears at ag-factor of greater than or equal to 2.037 and less than or equal to2.039, a second signal that appears at a g-factor of greater than orequal to 2.001 and less than or equal to 2.003, and a third signal thatappears at a g-factor of greater than or equal to 1.964 and less than orequal to 1.966 are observed. The split width of the first and secondsignals and the split width of the second and third signals, which areobtained by ESR measurement using an X-band, are each approximately 5mT. The sum of the spin densities of the first signal that appears at ag-factor of greater than or equal to 2.037 and less than or equal to2.039, the second signal that appears at a g-factor of greater than orequal to 2.001 and less than or equal to 2.003, and the third signalthat appears at a g-factor of greater than or equal to 1.964 and lessthan or equal to 1.966 is less than 1×10¹⁸ spins/cm³, typically greaterthan or equal to 1×10¹⁷ spins/cm³ and less than 1×10¹⁸ spins/cm³.

In the ESR spectrum at 100 K or lower, the first signal that appears ata g-factor of greater than or equal to 2.037 and less than or equal to2.039, the second signal that appears at a g-factor of greater than orequal to 2.001 and less than or equal to 2.003, and the third signalthat appears at a g-factor of greater than or equal to 1.964 and lessthan or equal to 1.966 correspond to signals attributed to nitrogenoxide (NO_(x); x is greater than 0 and less than or equal to 2,preferably greater than or equal to 1 and less than or equal to 2).Typical examples of nitrogen oxide include nitrogen monoxide andnitrogen dioxide. In other words, the smaller the sum of the spindensities of the first signal that appears at a g-factor greater than orequal to 2.037 and less than or equal to 2.039, the second signal thatappears at a g-factor greater than or equal to 2.001 and less than orequal to 2.003, and the third signal that appears at a g-factor greaterthan or equal to 1.964 and less than or equal to 1.966 is, the lower thecontent of nitrogen oxide in the oxide insulating film is.

The nitrogen concentration of the oxide insulating film measured by SIMSis lower than or equal to 6×10²⁰ atoms/cm³.

The oxide insulating film is formed by a PECVD method at a substratetemperature higher than or equal to 220° C. and lower than or equal to350° C. using silane and dinitrogen monoxide, whereby a dense and hardfilm can be formed.

The insulating film 116 in contact with the insulating film 114 isformed using an oxide insulating film whose oxygen content is in excessof that in the stoichiometric composition. Part of oxygen is releasedfrom the oxide insulating film whose oxygen content is in excess of thatin the stoichiometric composition by heating. The oxide insulating filmwhose oxygen content is in excess of that in the stoichiometriccomposition is an oxide insulating film of which the amount of releasedoxygen converted into oxygen atoms is greater than or equal to 1.0×10¹atoms/cm³, preferably greater than or equal to 3.0×10²⁰ atoms/cm³ in TDSanalysis. Note that the temperature of the film surface in the TDSanalysis is preferably higher than or equal to 100° C. and lower than orequal to 700° C., or higher than or equal to 100° C. and lower than orequal to 500° C.

Furthermore, it is preferable that the amount of defects in theinsulating film 116 be small, typically the spin density of a signalthat appears at g=2.001 due to a dangling bond of silicon be less than1.5×10¹⁸ spins/cm³, preferably less than or equal to 1×10¹⁸ spins/cm³ byESR measurement. Note that the insulating film 116 is provided moreapart from the oxide semiconductor film 223 than the insulating film 114is; thus, the insulating film 116 may have higher defect density thanthe insulating film 114.

The thickness of the insulating film 114 can be greater than or equal to5 nm and less than or equal to 150 nm, preferably greater than or equalto 5 nm and less than or equal to 50 nm, more preferably greater than orequal to 10 nm and less than or equal to 30 nm. The thickness of theinsulating film 116 can be greater than or equal to 30 nm and less thanor equal to 500 nm, preferably greater than or equal to 150 nm and lessthan or equal to 400 nm.

The insulating films 114 and 116 can be formed using insulating filmsformed of the same kinds of materials; thus, a boundary between theinsulating films 114 and 116 cannot be clearly observed in some cases.Thus, in this embodiment, the boundary between the insulating films 114and 116 is shown by a dashed line. Although a two-layer structure of theinsulating films 114 and 116 is described in this embodiment, thepresent invention is not limited to this. For example, a single-layerstructure of the insulating film 114, a single-layer structure of theinsulating film 116, or a stacked-layer structure of three or morelayers may be used.

Heat treatment (hereinafter referred to as first heat treatment) ispreferably performed after the insulating films 114 and 116 are formed.The first heat treatment can reduce nitrogen oxide included in theinsulating films 114 and 116. By the first heat treatment, part ofoxygen included in the insulating films 114 and 116 can be moved to theoxide semiconductor film 223, so that the number of oxygen vacanciesincluded in the oxide semiconductor film 223 can be reduced.

The temperature of the first heat treatment is typically lower than 400°C., preferably lower than 375° C., further preferably higher than orequal to 150° C. and lower than or equal to 350° C. The first heattreatment may be performed under an atmosphere of nitrogen, oxygen,ultra-dry air (air with a water content of 20 ppm or less, preferably 1ppm or less, more preferably 10 ppb or less), or a rare gas (argon,helium, or the like). The atmosphere of nitrogen, oxygen, ultra-dry air,or a rare gas preferably does not contain hydrogen, water, and the like.An electric furnace, a rapid thermal annealing (RTA) apparatus, or thelike can be used for the heat treatment.

The opening 141 is formed to expose part of the drain electrode 225 b.The opening 141 can be formed by a dry etching method, for example.Alternatively, a wet etching method or a combination of dry etching andwet etching can be employed for formation of the opening 141. Note thatthe etching step of forming the opening 141 can reduce the thickness ofthe drain electrode 225 b in some cases.

Next, an oxide semiconductor film to be the oxide conductor film 227 isformed over the insulating film 116 to cover the opening 141 (FIGS. 18Aand 18B).

Note that FIG. 18A is a schematic cross-sectional view of the inside ofa deposition apparatus when the oxide semiconductor film is formed overthe insulating film 116. In FIG. 18A, a sputtering apparatus is used asthe deposition apparatus, and a target 193 placed inside the sputteringapparatus and plasma 194 formed under the target 193 are schematicallyshown.

When the oxide semiconductor film is formed, plasma discharge isperformed in an atmosphere containing a third oxygen gas. At this time,oxygen is added to the insulating film 116 over which the oxidesemiconductor film is to be formed. When the oxide semiconductor film isformed, an inert gas (e.g., a helium gas, an argon gas, or a xenon gas)and the third oxygen gas may be mixed. For example, it is preferable touse the argon gas and the third oxygen gas with the flow rate of thethird oxygen gas higher than the flow rate of the argon gas. When theflow rate of the third oxygen gas is set higher, oxygen can be favorablyadded to the insulating film 116. As an example of the formationconditions of the oxide semiconductor film, the proportion of the thirdoxygen gas in a whole deposition gas can be higher than or equal to 50%and lower than or equal to 100%, preferably higher than or equal to 80%and lower than or equal to 100%.

In FIG. 18A, oxygen or excess oxygen added to the insulating film 116 isschematically shown by arrows of broken lines.

The oxide semiconductor film is formed at a substrate temperature higherthan or equal to room temperature and lower than 340° C., preferablyhigher than or equal to room temperature and lower than or equal to 300°C., further preferably higher than or equal to 100° C. and lower than orequal to 250° C., still further preferably higher than or equal to 100°C. and lower than or equal to 200° C. The oxide semiconductor film isformed while being heated, so that the crystallinity of the oxidesemiconductor film can be increased. On the other hand, in the casewhere a large-sized glass substrate (e.g., the 6th generation to the10th generation) is used as the substrate 211 and the oxidesemiconductor film is formed at a substrate temperature higher than orequal to 150° C. and lower than 340° C., the substrate 211 might bechanged in shape (distorted or warped). In the case where a large-sizedglass substrate is used, the change in the shape of the glass substratecan be suppressed by forming the oxide semiconductor film at a substratetemperature higher than or equal to 100° C. and lower than 150° C.

In this embodiment, the oxide semiconductor film is formed by asputtering method using an In—Ga—Zn metal oxide target (withIn:Ga:Zn=1:3:6 [atomic ratio]).

Next, the oxide semiconductor film is processed into a desired shape toform an island-shaped oxide semiconductor film 227 a (see FIG. 18C).

The oxide semiconductor film 227 a can be formed in such a manner thatan oxide semiconductor film is formed over the insulating film 116, theoxide semiconductor film is patterned so that a desired region thereofremains, and then unnecessary regions are etched.

Next, the insulating film 217 is formed over the insulating film 116 andthe oxide semiconductor film 227 a (see FIG. 19A).

The insulating film 217 has a function of blocking oxygen, hydrogen,water, alkali metal, alkaline earth metal, or the like. With theinsulating film 217, diffusion of oxygen from the oxide semiconductorfilm 223 to the outside, diffusion of oxygen contained in the insulatingfilm 215 to the outside, and entry of hydrogen, water, alkali metal,alkaline earth metal, or the like from the outside into the oxidesemiconductor film 223 can be prevented.

The insulating film 217 preferably contains one or both of hydrogen andnitrogen. As the insulating film 217, a silicon nitride film ispreferably used, for example. The insulating film 217 can be formed by asputtering method or a PECVD method, for example. In the case where theinsulating film 217 is formed by a PECVD method, for example, thesubstrate temperature is lower than 400° C., preferably lower than 375°C., further preferably higher than or equal to 180° C. and lower than orequal to 350° C. The substrate temperature at which the insulating film217 is formed is preferably within the above range because a dense filmcan be formed. Furthermore, when the substrate temperature at which theinsulating film 217 is formed is within the above range, oxygen orexcess oxygen in the insulating films 114 and 116 can be moved to theoxide semiconductor film 223.

Note that instead of the nitride insulating film having a blockingeffect against oxygen, hydrogen, water, alkali metal, alkaline earthmetal, and the like, an oxide insulating film having a blocking effectagainst oxygen, hydrogen, water, and the like, may be provided. As theoxide insulating film having a blocking effect against oxygen, hydrogen,water, and the like, an aluminum oxide film, an aluminum oxynitridefilm, a gallium oxide film, a gallium oxynitride film, an yttrium oxidefilm, an yttrium oxynitride film, a hafnium oxide film, and a hafniumoxynitride film can be given.

After the insulating film 217 is formed, heat treatment similar to thefirst heat treatment (hereinafter referred to as second heat treatment)may be performed. Through such heat treatment at lower than 400° C.,preferably lower than 375° C., further preferably higher than or equalto 180° C. and lower than or equal to 350° C. after the addition ofoxygen to the insulating film 116 when the oxide semiconductor film tobe the oxide conductor film 227 is formed, oxygen or excess oxygen inthe insulating film 116 can be moved into the oxide semiconductor film223 and compensate oxygen vacancies in the oxide semiconductor film 223.

Oxygen moved to the oxide semiconductor film 223 is described withreference to FIG. 20. FIG. 20 are model diagrams illustrating oxygenmoved to the oxide semiconductor film 223 due to the substratetemperature at the time of forming the insulating film 217 (typically,lower than 375° C.) or the second heat treatment after the formation ofthe insulating film 217 (typically, lower than 375° C.). In FIG. 20,oxygen (oxygen radicals, oxygen atoms, or oxygen molecules) moved to theoxide semiconductor film 223 is shown by arrows of broken lines. Notethat FIG. 20 is a cross-sectional view of the transistor after theinsulating film 217 is formed and its periphery.

In the oxide semiconductor film 223 in FIG. 20, oxygen vacancies arecompensated with oxygen moved from films in contact with the oxidesemiconductor film 223 (here, the insulating film 107 and the insulatingfilm 114). Specifically, in the input/output device of one embodiment ofthe present invention, the insulating film 107 includes an excess oxygenregion because an oxygen gas is used at the time of forming the oxidesemiconductor film to be the oxide semiconductor film 223 by sputteringand oxygen is added to the insulating film 107. Furthermore, theinsulating film 116 includes an excess oxygen region because an oxygengas is used at the time of forming the oxide semiconductor film to bethe oxide conductor film 227 by sputtering and oxygen is added to theinsulating film 116. In the oxide semiconductor film 223 between theinsulating films including the excess oxygen regions, oxygen vacanciescan be favorably compensated.

Furthermore, the insulating film 106 is provided under the insulatingfilm 107, and the insulating film 217 is provided over the insulatingfilms 114 and 116. When the insulating films 106 and 217 are formedusing a material having low oxygen permeability, e.g., silicon nitride,oxygen contained in the insulating films 107, 114, and 116 can beconfined to the oxide semiconductor film 223 side; thus, oxygen can befavorably moved to the oxide semiconductor film 223.

The insulating film 217 preferably has a function of lowering theresistivity of the oxide conductor film 227.

With the insulating film 217 containing one or both of hydrogen andnitrogen, one or both of hydrogen and nitrogen is added to the oxidesemiconductor film 227 a in contact with the insulating film 217.Accordingly, the carrier density of the oxide semiconductor film 227 ais increased, and the oxide semiconductor film 227 a can serve as anoxide conductor film.

Note that the oxide semiconductor film 227 a with decreased resistivityis illustrated as the oxide conductor film 227 after FIG. 19A.

The resistivity of the oxide conductor film 227 is lower than at leastthe resistivity of the oxide semiconductor film 223 and is preferablyhigher than or equal to 1×10⁻³ Ωcm and lower than 1×10⁴ Ωcm, furtherpreferably higher than or equal to 1×10⁻³ Ωcm and lower than 1×10⁻¹ Ωcm.

Then, an opening 142 is formed as follows: the insulating film 219 isformed over the insulating film 217, the insulating films 217 and 219are patterned so that a desired region thereof remains, and unnecessaryregions are etched (see FIG. 19B).

In this embodiment, an acrylic resin is used for the insulating film219.

The opening 142 is formed to expose the drain electrode 225 b. Theopening 142 can be formed by a dry etching method, for example.Alternatively, a wet etching method or a combination of dry etching andwet etching can be employed for formation of the opening 142. Note thatthe etching step of forming the opening 142 can reduce the thickness ofthe drain electrode 225 b in some cases.

Note that the opening may be formed in the insulating films 114, 116,217, and 219 at one time in the step of forming the opening 142 withoutperforming the step of forming the opening 141. In this case, the numberof steps of manufacturing the input/output device of one embodiment ofthe present invention is reduced, resulting in a reduction of themanufacturing cost.

Then, a conductive film is formed over the insulating film 219 to coverthe opening 142 and is patterned so that a desired region thereofremains, and unnecessary regions are etched; thus, the conductive film251 is formed. The insulating film 253 is formed over the conductivefilm 251. Then, a conductive film is formed over the insulating film 253and is patterned so that a desired region of the conductive filmremains, and an unnecessary region is etched; thus, the conductive film255 is formed. After that, a conductive film is formed over theinsulating film 253 and the conductive film 255 and is patterned so thata desired region of the conductive film remains, and then, anunnecessary region is etched to form the conductive film 252 (see FIG.19C).

In this embodiment, ITO films are used as the conductive films 251 and252, a silicon nitride film is used as the insulating film 253, an alloyfilm of silver, palladium, and copper (also referred to as Ag—Pd—Cu orAPC) is used as the conductive film 255.

The formation order of the conductive films 252 and 255 is not limited;however, the conductive film 255 is preferably formed before theconductive film 252. In such a case, damage to the conductive film 252caused by etching of the conductive film 255 can be reduced, forexample.

Note that the conductive film 251 may be formed using an oxidesemiconductor film by a method similar to that of the oxide conductorfilm 227. In this case, the insulating film 253 over the conductive film251 can be formed using a material that can be used for the insulatingfilm 217. The conductive film 252 may be formed by forming an oxidesemiconductor film and performing treatment for lowering the resistivityof the oxide semiconductor film.

Through the above steps, the transistor 203 b and the pair of electrodesof the liquid crystal element illustrated in FIG. 4 can be formed.

Note that although the structure with the insulating film 219 is shownin FIG. 19C, a structure without the insulating film 219 may be employed(see FIG. 21).

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 3

In this embodiment, a transistor that can be used for the input/outputdevice of one embodiment of the present invention is described withreference to FIGS. 22A to 22C, FIGS. 23A to 23D, FIGS. 24A and 24B, andFIGS. 25A to 25D. Note that the description in Embodiment 1 can bereferred to for the material of each layer.

<Structure Example 1 of Transistor>

FIG. 22A is a top view of a transistor 270. FIG. 22B is across-sectional view taken along the dashed-dotted line A1-A2 in FIG.22A, and FIG. 22C is a cross-sectional view taken along thedashed-dotted line B1-B2 in FIG. 22A. Note that the direction of thedashed dotted line A1-A2 may be called the channel length direction, andthe direction of the dashed dotted line B1-B2 may be called the channelwidth direction.

The transistor 270 includes a conductive film 504 serving as a firstgate electrode over a substrate 502, an insulating film 506 over thesubstrate 502 and the conductive film 504, an insulating film 507 overthe insulating film 506, an oxide semiconductor film 508 over theinsulating film 507, a conductive film 512 a serving as a sourceelectrode electrically connected to the oxide semiconductor film 508, aconductive film 512 b serving as a drain electrode electricallyconnected to the oxide semiconductor film 508, insulating films 514 and516 over the oxide semiconductor film 508 and the conductive films 512 aand 512 b, and an oxide conductor film 511 b over the insulating film516. In addition, an insulating film 518 is provided over the oxideconductor film 511 b.

In the transistor 270, the insulating films 514 and 516 serve as asecond gate insulating film of the transistor 270. An oxidesemiconductor film 511 a is connected to the conductive film 512 bthrough an opening 552 c provided in the insulating films 514 and 516.The oxide semiconductor film 511 a serves as, for example, a pixelelectrode of a display element. The oxide conductor film 511 b in thetransistor 270 serves as a second gate electrode (also referred to as aback gate electrode).

As illustrated in FIG. 22C, the oxide conductor film 511 b is connectedto the conductive film 504 serving as a first gate electrode throughopenings 552 a and 552 b provided in the insulating films 506, 507, 514,and 516. Accordingly, the oxide conductor film 504 and the oxideconductor film 511 b are supplied with the same potential.

Note that although the structure in which the openings 552 a and 552 bare provided so that the oxide conductor film 511 b and the conductivefilm 504 are connected to each other is described in this embodiment,one embodiment of the present invention is not limited thereto. Forexample, a structure in which only one of the openings 552 a and 552 bis provided so that the oxide conductor film 511 b and the conductivefilm 504 are connected to each other, or a structure in which theopenings 552 a and 552 b are not provided and the oxide conductor film511 b and the conductive film 504 are not connected to each other may beemployed. Note that in the case where the oxide conductor film 511 b andthe conductive film 504 are not connected to each other, it is possibleto apply different potentials to the oxide conductor film 511 b and theconductive film 504.

As illustrated in FIG. 22B, the oxide semiconductor film 508 ispositioned to face each of the conductive film 504 serving as a firstgate electrode and the oxide conductor film 511 b serving as a secondgate electrode, and is sandwiched between the two conductive filmsserving as gate electrodes. The length in the channel length directionand the length in the channel width direction of the oxide conductorfilm 511 b serving as a second gate electrode are longer than that inthe channel length direction and that in the channel width direction ofthe oxide semiconductor film 508, respectively. The whole oxidesemiconductor film 508 is covered with the oxide conductor film 511 bwith the insulating films 514 and 516 positioned therebetween. Since theoxide conductor film 511 b serving as a second gate electrode isconnected to the conductive film 504 serving as a first gate electrodethrough the openings 552 a and 552 b provided in the insulating films506 and 507 and the insulating films 514 and 516, a side surface of theoxide semiconductor film 508 in the channel width direction faces theoxide conductor film 511 b serving as a second gate electrode with theinsulating films 514 and 516 positioned therebetween.

In other words, in the channel width direction of the transistor 270,the conductive film 504 serving as a first gate electrode and the oxideconductor film 511 b serving as a second gate electrode are connected toeach other through the openings provided in the insulating films 506 and507 serving as gate insulating films and the insulating films 514 and516 serving as second gate insulating films; and the conductive film 504and the oxide conductor film 511 b surround the oxide semiconductor film508 with the insulating films 506 and 507 serving as the gate insulatingfilms and the insulating films 514 and 516 serving as the second gateinsulating films positioned therebetween.

Such a structure enables the oxide semiconductor film 508 included inthe transistor 270 to be electrically surrounded by electric fields ofthe conductive film 504 serving as a first gate electrode and the oxideconductor film 511 b serving as a second gate electrode. A devicestructure of a transistor, like that of the transistor 270, in whichelectric fields of a first gate electrode and a second gate electrodeelectrically surround an oxide semiconductor film where a channel regionis formed, can be referred to as a surrounded channel (s-channel)structure.

Since the transistor 270 has the s-channel structure, an electric fieldfor inducing a channel can be effectively applied to the oxidesemiconductor film 508 by the conductive film 504 serving as a firstgate electrode; therefore, the current drive capability of thetransistor 270 can be improved and high on-state current characteristicscan be obtained. Since the on-state current can be increased, it ispossible to reduce the size of the transistor 270. In addition, sincethe transistor 270 is surrounded by the conductive film 504 serving as afirst gate electrode and the oxide conductor film 511 b serving as asecond gate electrode, the mechanical strength of the transistor 270 canbe increased.

<Structure Example 2 of Transistor>

FIGS. 23A and 23B illustrate a cross-sectional view illustrating amodification example of the transistor 270 in FIGS. 22B and 22C. FIGS.23C and 23D illustrate a cross-sectional view illustrating anothermodification example of the transistor 270 in FIGS. 22B and 22C.

A transistor 270A in FIGS. 23A and 23B is different from the transistor270 in FIGS. 22B and 22C in that the oxide semiconductor film 508 has athree-layer structure. Specifically, the oxide semiconductor film 508 ofthe transistor 270A includes an oxide semiconductor film 508 a, an oxidesemiconductor film 508 b, and an oxide semiconductor film 508 c.

A transistor 270B in FIGS. 23C and 23D is different from the transistor270 in FIGS. 22B and 22C in that the oxide semiconductor film 508 has atwo-layer structure. Specifically, the oxide semiconductor film 508 ofthe transistor 270B includes the oxide semiconductor films 508 b and 508c.

Here, a band structure including the oxide semiconductor film 508 andinsulating films in contact with the oxide semiconductor film 508 isdescribed with reference to FIGS. 24A and 24B.

FIG. 24A shows an example of a band structure in the thickness directionof a layered structure including the insulating film 507, the oxidesemiconductor films 508 a, 508 b, and 508 c, and the insulating film514. FIG. 24B shows an example of a band structure in the thicknessdirection of a layered structure including the insulating film 507, theoxide semiconductor films 508 b and 508 c, and the insulating film 514.For easy understanding, the energy level of the conduction band minimum(Ec) of each of the insulating film 507, the oxide semiconductor films508 a, 508 b, and 508 c, and the insulating film 514 is shown in theband structures.

In the band structure of FIG. 24A, a silicon oxide film is used as eachof the insulating film 507 and the insulating film 514, an oxidesemiconductor film formed using a metal oxide target having an atomicratio of metal elements, In:Ga:Zn=1:1:1.2, is used as the oxidesemiconductor film 508 a, an oxide semiconductor film formed using ametal oxide target having an atomic ratio of metal elements,In:Ga:Zn=4:2:4.1, is used as the oxide semiconductor film 508 b, and anoxide semiconductor film formed using a metal oxide target having anatomic ratio of metal elements, In:Ga:Zn=1:1:1.2, is used as the oxidesemiconductor film 508 c.

In the band structure of FIG. 24B, a silicon oxide film is used as eachof the insulating film 507 and the insulating film 514, an oxidesemiconductor film formed using a metal oxide target having an atomicratio of metal elements, In:Ga:Zn=4:2:4.1, is used as the oxidesemiconductor film 508 b, and an oxide semiconductor film formed using ametal oxide target having an atomic ratio of metal elements,In:Ga:Zn=1:1:1.2, is used as the oxide semiconductor film 508 c.

As illustrated in FIGS. 24A and 24B, the energy level of the conductionband minimum gradually changes between the oxide semiconductor films 508a and 508 b and between the oxide semiconductor films 508 b and 508 c.In other words, the energy level of the conduction band minimum iscontinuously changed or continuously connected. To obtain such a bandstructure, there exists no impurity, which forms a defect state such asa trap center or a recombination center, at the interface between theoxide semiconductor films 508 a and 508 b or at the interface betweenthe oxide semiconductor films 508 b and 508 c.

To form a continuous junction between the oxide semiconductor films 508a and 508 b and between the oxide semiconductor films 508 b and 508 c,it is necessary to form the films successively without exposure to theair by using a multi-chamber deposition apparatus (sputtering apparatus)provided with a load lock chamber.

With the band structures of FIGS. 24A and 24B, the oxide semiconductorfilm 508 b serves as a well, and a channel region is formed in the oxidesemiconductor film 508 b in the transistor with the layered structure.

By providing the oxide semiconductor films 508 a and 508 c, the oxidesemiconductor film 508 b can be distanced away from trap states.

In addition, the trap states might be more distant from the vacuum levelthan the energy level of the conduction band minimum (Ec) of the oxidesemiconductor film 508 b serving as a channel region, so that electronsare likely to be accumulated in the trap states. When the electrons areaccumulated in the trap states, the electrons become negative fixedelectric charge, so that the threshold voltage of the transistor isshifted in the positive direction. Therefore, it is preferable that thetrap states be closer to the vacuum level than the energy level of theconduction band minimum (Ec) of the oxide semiconductor film 508 b. Sucha structure inhibits accumulation of electrons in the trap states. As aresult, the on-state current and the field-effect mobility of thetransistor can be increased.

The energy level of the conduction band minimum of each of the oxidesemiconductor films 508 a and 508 c is closer to the vacuum level thanthat of the oxide semiconductor film 508 b. Typically, a difference inenergy level between the conduction band minimum of the oxidesemiconductor film 508 b and the conduction band minimum of each of theoxide semiconductor films 508 a and 508 c is 0.15 eV or more or 0.5 eVor more and 2 eV or less or 1 eV or less. That is, the differencebetween the electron affinity of each of the oxide semiconductor films508 a and 508 c and the electron affinity of the oxide semiconductorfilm 508 b is 0.15 eV or more or 0.5 eV or more and 2 eV or less or 1 eVor less.

In such a structure, the oxide semiconductor film 508 b serves as a mainpath of a current. In other words, the oxide semiconductor film 508 bserves as a channel region, and the oxide semiconductor films 508 a and508 c serve as oxide insulating films. In addition, since the oxidesemiconductor films 508 a and 508 c each include one or more metalelements included in the oxide semiconductor film 508 b in which achannel region is formed, interface scattering is less likely to occurat the interface between the oxide semiconductor films 508 a and 508 bor at the interface between the oxide semiconductor films 508 b and 508c. Thus, the transistor can have high field-effect mobility because themovement of carriers is not hindered at the interface.

To prevent each of the oxide semiconductor films 508 a and 508 c fromserving as part of a channel region, a material having sufficiently lowconductivity is used for the oxide semiconductor films 508 a and 508 c.Thus, the oxide semiconductor films 508 a and 508 c can be referred toas oxide insulating films for such properties and/or functions.Alternatively, a material that has a smaller electron affinity (adifference in energy level between the vacuum level and the conductionband minimum) than the oxide semiconductor film 508 b and has adifference in energy level in the conduction band minimum from the oxidesemiconductor film 508 b (band offset) is used for the oxidesemiconductor films 508 a and 508 c. Furthermore, to inhibit generationof a difference in threshold voltage due to the value of the drainvoltage, the energy level of the conduction band minimum of each of theoxide semiconductor films 508 a and 508 c is preferably closer to thevacuum level than the energy level of the conduction band minimum of theoxide semiconductor film 508 b is. For example, a difference between theenergy level of the conduction band minimum of the oxide semiconductorfilm 508 b and the energy level of the conduction band minimum of eachof the oxide semiconductor films 508 a and 508 c is preferably greaterthan or equal to 0.2 eV, more preferably greater than or equal to 0.5eV.

It is preferable that the oxide semiconductor films 508 a and 508 c nothave a spinel crystal structure. This is because if the oxidesemiconductor films 508 a and 508 c have a spinel crystal structure,constituent elements of the conductive films 512 a and 512 b might bediffused to the oxide semiconductor film 508 b at the interface betweenthe spinel crystal structure and another region. Note that each of theoxide semiconductor films 508 a and 508 c is preferably a CAAC-OS film,in which case a higher blocking property against constituent elements ofthe conductive films 512 a and 512 b, for example, copper elements, canbe obtained.

The thickness of each of the oxide semiconductor films 508 a and 508 cis greater than or equal to a thickness that is capable of inhibitingdiffusion of the constituent elements of the conductive films 512 a and512 b to the oxide semiconductor film 508 b, and less than a thicknessthat inhibits supply of oxygen from the insulating film 514 to the oxidesemiconductor film 508 b. For example, when the thickness of each of theoxide semiconductor films 508 a and 508 c is greater than or equal to 10nm, diffusion of the constituent elements of the conductive films 512 aand 512 b to the oxide semiconductor film 508 b can be inhibited. Whenthe thickness of each of the oxide semiconductor films 508 a and 508 cis less than or equal to 100 nm, oxygen can be effectively supplied fromthe insulating film 514 to the oxide semiconductor film 508 b.

Although the example where an oxide semiconductor film formed using ametal oxide target having an atomic ratio of metal elements,In:Ga:Zn=1:1:1.2, is used as each of the oxide semiconductor films 508 aand 508 c is described in this embodiment, one embodiment of the presentinvention is not limited thereto. For example, an oxide semiconductorfilm formed using a metal oxide target having an atomic ratio ofIn:Ga:Zn=1:1:1, In:Ga:Zn=1:3:2, In:Ga:Zn=1:3:4, or In:Ga:Zn=1:3:6, maybe used as each of the oxide semiconductor films 508 a and 508 c.

When the oxide semiconductor films 508 a and 508 c are formed using ametal oxide target having an atomic ratio of In:Ga:Zn=1:1:1, the oxidesemiconductor films 508 a and 508 c have an atomic ratio ofIn:Ga:Zn=1:β1 (0<β1≤2):β2 (0<β2≤3) in some cases. When the oxidesemiconductor films 508 a and 508 c are formed using a metal oxidetarget having an atomic ratio of In:Ga:Zn=1:3:4, the oxide semiconductorfilms 508 a and 508 c have an atomic ratio of In:Ga:Zn=1:β3 (1≤β3≤5):β4(2≤β4≤6) in some cases. When the oxide semiconductor films 508 a and 508c are formed using a metal oxide target having an atomic ratio ofIn:Ga:Zn=1:3:6, the oxide semiconductor films 508 a and 508 c have anatomic ratio of In:Ga:Zn=1:β5 (1≤β5≤5):β6 (4≤β6≤8) in some cases.

The drawings illustrate an example where the oxide semiconductor film508 in the transistor 270 and the oxide semiconductor film 508 c in thetransistors 270A and 270B have a small thickness in a region that doesnot overlap with the conductive films 512 a and 512 b, that is, anexample where part of the oxide semiconductor film has a depressedportion. However, one embodiment of the present invention is not limitedthereto, and the oxide semiconductor film does not necessarily have adepressed region in a region that does not overlap with the conductivefilms 512 a and 512 b. FIGS. 25A and 25B illustrate examples in thiscase. FIGS. 25A and 25B are cross-sectional views illustrating anexample of the transistor. FIGS. 25A and 25B illustrate a structurewhere the oxide semiconductor film 508 in the transistor 270B does nothave a depressed portion.

As illustrated in FIGS. 25C and 25D, the oxide semiconductor film 508 cmay be formed thinner than the oxide semiconductor film 508 b inadvance, and an insulating film 519 may further be formed over the oxidesemiconductor film 508 c and the insulating film 507. In that case,openings for connecting the oxide semiconductor film 508 c and theconductive films 512 a and 512 b are formed in the insulating film 519.The insulating film 519 can be formed with the same material and thesame forming method as the insulating film 514.

The structures of the transistors of this embodiment can be freelycombined with each other.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 4

In this embodiment, an oxide semiconductor is described with referenceto FIGS. 26A to 26E, FIGS. 27A to 27E, FIGS. 28A to 28D, FIGS. 29A and29B, and FIG. 30.

<Structure of Oxide Semiconductor>

The structure of an oxide semiconductor is described below.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a c-axis-alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor.Examples of a crystalline oxide semiconductor include a single crystaloxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor,and an nc-OS.

An amorphous structure is generally thought to be isotropic and have nonon-uniform structure, to be metastable and not have fixed positions ofatoms, to have a flexible bond angle, and to have a short-range orderbut have no long-range order, for example.

This means that a stable oxide semiconductor cannot be regarded as acompletely amorphous oxide semiconductor. Moreover, an oxidesemiconductor that is not isotropic (e.g., an oxide semiconductor thathas a periodic structure in a microscopic region) cannot be regarded asa completely amorphous oxide semiconductor. In contrast, an a-like OS,which is not isotropic, has an unstable structure that contains a void.Because of its instability, an a-like OS is close to an amorphous oxidesemiconductor in terms of physical properties.

<CAAC-OS>

First, a CAAC-OS is described.

A CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

Analysis of a CAAC-OS by X-ray diffraction (XRD) is described. Forexample, when the structure of a CAAC-OS including an InGaZnO₄ crystalthat is classified into the space group R-3m is analyzed by anout-of-plane method, a peak appears at a diffraction angle (2θ) ofaround 310 as shown in FIG. 26A. This peak is derived from the (009)plane of the InGaZnO₄ crystal, which indicates that crystals in theCAAC-OS have c-axis alignment, and that the c-axes are aligned in adirection substantially perpendicular to a surface over which theCAAC-OS film is formed (also referred to as a formation surface) or thetop surface of the CAAC-OS film. Note that a peak sometimes appears at a2θ of around 36° in addition to the peak at a 2θ of around 31°. The peakat a 2θ of around 36° is derived from a crystal structure that isclassified into the space group Fd-3m; thus, it is preferable that thispeak not appear in a CAAC-OS.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray is incident on the CAAC-OS in a directionparallel to the formation surface, a peak appears at a 2θ of around 56°.This peak is attributed to the (110) plane of the InGaZnO₄ crystal. Whenanalysis (ϕ scan) is performed with 2θ fixed at around 56° and with thesample rotated using a normal vector to the sample surface as an axis (ϕaxis), as shown in FIG. 26B, a peak is not clearly observed. Incontrast, in the case where single crystal InGaZnO₄ is subjected to ϕscan with 2θ fixed at around 56°, as shown in FIG. 26C, six peaks thatare derived from crystal planes equivalent to the (110) plane areobserved. Accordingly, the structural analysis using XRD shows that thedirections of a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the formation surface of the CAAC-OS, a diffraction pattern(also referred to as a selected-area electron diffraction pattern) shownin FIG. 26D can be obtained. In this diffraction pattern, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are included. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OShave c-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 26E shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 26E, a ring-like diffraction pattern isobserved. Thus, the electron diffraction using an electron beam with aprobe diameter of 300 nm also indicates that the a-axes and b-axes ofthe pellets included in the CAAC-OS do not have regular orientation. Thefirst ring in FIG. 26E is considered to be derived from the (010) plane,the (100) plane, and the like of the InGaZnO₄ crystal. The second ringin FIG. 26E is considered to be derived from the (110) plane and thelike.

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, even in thehigh-resolution TEM image, a boundary between pellets, that is, a grainboundary is not clearly observed in some cases. Thus, in the CAAC-OS, areduction in electron mobility due to the grain boundary is less likelyto occur.

FIG. 27A shows a high-resolution TEM image of a cross section of theCAAC-OS that is observed from a direction substantially parallel to thesample surface. The high-resolution TEM image is obtained with aspherical aberration corrector function. The high-resolution TEM imageobtained with a spherical aberration corrector function is particularlyreferred to as a Cs-corrected high-resolution TEM image. TheCs-corrected high-resolution TEM image can be observed with, forexample, an atomic resolution analytical electron microscope JEM-ARM200Fmanufactured by JEOL Ltd.

FIG. 27A shows pellets in which metal atoms are arranged in a layeredmanner. FIG. 27A proves that the size of a pellet is greater than orequal to 1 nm or greater than or equal to 3 nm. Therefore, the pelletcan also be referred to as a nanocrystal (nc). Furthermore, the CAAC-OScan also be referred to as an oxide semiconductor including c-axisaligned nanocrystals (CANC). A pellet reflects unevenness of a formationsurface or a top surface of the CAAC-OS, and is parallel to theformation surface or the top surface of the CAAC-OS.

FIGS. 27B and 27C show Cs-corrected high-resolution TEM images of aplane of the CAAC-OS observed from a direction substantiallyperpendicular to the sample surface. FIGS. 27D and 27E are imagesobtained through image processing of FIGS. 27B and 27C. The method ofimage processing is as follows. The image in FIG. 27B is subjected tofast Fourier transform (FFT), so that an FFT image is obtained. Then,mask processing is performed such that a range of from 2.8 nm⁻¹ to 5.0nm⁻¹ from the origin in the obtained FFT image remains. After the maskprocessing, the FFT image is processed by inverse fast Fourier transform(IFFT) to obtain a processed image. The image obtained in this manner iscalled an FFT filtering image. The FFT filtering image is a Cs-correctedhigh-resolution TEM image from which a periodic component is extracted,and shows a lattice arrangement.

In FIG. 27D, a portion where a lattice arrangement is broken is denotedwith a dashed line. A region surrounded by a dashed line is one pellet.The portion denoted with the dashed line is a junction of pellets. Thedashed line draws a hexagon, which means that the pellet has a hexagonalshape. Note that the shape of the pellet is not always a regular hexagonbut is a non-regular hexagon in many cases.

In FIG. 27E, a dotted line denotes a portion between a region where alattice arrangement is well aligned and another region where a latticearrangement is well aligned. A clear crystal grain boundary cannot beobserved even in the vicinity of the dotted line. When a lattice pointin the vicinity of the dotted line is regarded as a center andsurrounding lattice points are joined, a distorted hexagon, pentagon,and/or heptagon can be formed, for example. That is, a latticearrangement is distorted so that formation of a crystal grain boundaryis inhibited. This is probably because the CAAC-OS can toleratedistortion owing to a low density of the atomic arrangement in an a-bplane direction, the interatomic bond distance changed by substitutionof a metal element, and the like.

As described above, the CAAC-OS has c-axis alignment, its pellets(nanocrystals) are connected in an a-b plane direction, and the crystalstructure has distortion. For this reason, the CAAC-OS can also bereferred to as an oxide semiconductor including a c-axis-aligneda-b-plane-anchored (CAA) crystal.

The CAAC-OS is an oxide semiconductor with high crystallinity. Entry ofimpurities, formation of defects, or the like might decrease thecrystallinity of an oxide semiconductor. This means that the CAAC-OS hassmall amounts of impurities and defects (e.g., oxygen vacancies).

Note that the impurity means an element other than the main componentsof the oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically, siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities ordefects might be changed by light, heat, or the like. Impuritiesincluded in the oxide semiconductor might serve as carrier traps orcarrier generation sources, for example. For example, an oxygen vacancyin the oxide semiconductor might serve as a carrier trap or serve as acarrier generation source when hydrogen is captured therein.

The CAAC-OS having small amounts of impurities and oxygen vacancies isan oxide semiconductor film with a low carrier density (specifically,lower than 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³, and furtherpreferably lower than 1×10¹⁰/cm³ and higher than or equal to1×10⁻⁹/cm³). Such an oxide semiconductor is referred to as a highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor. A CAAC-OS has a low impurity concentration and a lowdensity of defect states. Thus, the CAAC-OS can be referred to as anoxide semiconductor having stable characteristics.

<nc-OS>

Next, an nc-OS is described.

Analysis of an nc-OS by XRD is described. For example, when thestructure of an nc-OS is analyzed by an out-of-plane method, a peakindicating orientation does not appear. That is, a crystal of an nc-OSdoes not have orientation.

For example, when an electron beam with a probe diameter of 50 nm isincident on a 34-nm-thick region of thinned nc-OS including an InGaZnO₄crystal in a direction parallel to the formation surface, a ring-shapeddiffraction pattern (nanobeam electron diffraction pattern) shown inFIG. 28A is observed. FIG. 28B shows a diffraction pattern (nanobeamelectron diffraction pattern) obtained when an electron beam with aprobe diameter of 1 nm is incident on the same sample. As shown in FIG.28B, a plurality of spots are observed in a ring-like region. In otherwords, ordering in an nc-OS is not observed with an electron beam with aprobe diameter of 50 nm but is observed with an electron beam with aprobe diameter of 1 nm.

Furthermore, an electron diffraction pattern in which spots are arrangedin an approximately hexagonal shape is observed in some cases as shownin FIG. 28C when an electron beam having a probe diameter of 1 nm isincident on a region with a thickness of less than 10 nm. This meansthat an nc-OS has a well-ordered region, i.e., a crystal, in the rangeof less than 10 nm in thickness. Note that an electron diffractionpattern having regularity is not observed in some regions becausecrystals are aligned in various directions.

FIG. 28D shows a Cs-corrected high-resolution TEM image of a crosssection of an nc-OS observed from the direction substantially parallelto the formation surface. In a high-resolution TEM image, an nc-OS has aregion in which a crystal part is observed, such as the part indicatedby additional lines in FIG. 28D, and a region in which a crystal part isnot clearly observed. In most cases, the size of a crystal part includedin the nc-OS is greater than or equal to 1 nm and less than or equal to10 nm, or specifically, greater than or equal to 1 nm and less than orequal to 3 nm. Note that an oxide semiconductor including a crystal partwhose size is greater than 10 nm and less than or equal to 100 nm issometimes referred to as a microcrystalline oxide semiconductor. In ahigh-resolution TEM image of the nc-OS, for example, a grain boundary isnot clearly observed in some cases. Note that there is a possibilitythat the origin of the nanocrystal is the same as that of a pellet in aCAAC-OS. Therefore, a crystal part of the nc-OS may be referred to as apellet in the following description.

As described above, in the nc-OS, a microscopic region (for example, aregion with a size greater than or equal to 1 nm and less than or equalto 10 nm, in particular, a region with a size greater than or equal to 1nm and less than or equal to 3 nm) has a periodic atomic arrangement.There is no regularity of crystal orientation between different pelletsin the nc-OS. Thus, the orientation of the whole film is not ordered.Accordingly, the nc-OS cannot be distinguished from an a-like OS or anamorphous oxide semiconductor, depending on an analysis method.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including random aligned nanocrystals (RANC) oran oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as comparedwith an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an a-like OS and an amorphousoxide semiconductor. Note that there is no regularity of crystalorientation between different pellets in the nc-OS. Therefore, the nc-OShas a higher density of defect states than the CAAC-OS.

<a-like OS>

An a-like OS has a structure between those of the nc-OS and theamorphous oxide semiconductor.

FIGS. 29A and 29B are high-resolution cross-sectional TEM images of ana-like OS. FIG. 29A is the high-resolution cross-sectional TEM image ofthe a-like OS at the start of the electron irradiation. FIG. 29B is thehigh-resolution cross-sectional TEM image of a-like OS after theelectron (e⁻) irradiation at 4.3×10⁸ e⁻/nm². FIGS. 29A and 29B show thatstripe-like bright regions extending vertically are observed in thea-like OS from the start of the electron irradiation. It can be alsofound that the shape of the bright region changes after the electronirradiation. Note that the bright region is presumably a void or alow-density region.

The a-like OS has an unstable structure because it contains a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS, an nc-OS, and a CAAC-OS are prepared as samples. Each ofthe samples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

It is known that a unit cell of an InGaZnO₄ crystal has a structure inwhich nine layers including three In—O layers and six Ga—Zn—O layers arestacked in the c-axis direction. The distance between the adjacentlayers is equivalent to the lattice spacing on the (009) plane (alsoreferred to as d value). The value is calculated to be 0.29 nm fromcrystal structural analysis. Accordingly, a portion where the spacingbetween lattice fringes is greater than or equal to 0.28 nm and lessthan or equal to 0.30 nm is regarded as a crystal part of InGaZnO₄ inthe following description. Each of lattice fringes corresponds to thea-b plane of the InGaZnO₄ crystal.

FIG. 30 shows the average size of crystal parts (at 22 points to 30points) in each sample. Note that the crystal part size corresponds tothe length of a lattice fringe. FIG. 30 indicates that the crystal partsize in the a-like OS increases with an increase in the cumulativeelectron dose in obtaining TEM images, for example. As shown in FIG. 30,a crystal part of approximately 1.2 nm (also referred to as an initialnucleus) at the start of TEM observation grows to a size ofapproximately 1.9 nm at a cumulative electron (e⁻) dose of 4.2×10⁸e⁻/nm². In contrast, the crystal part size in the nc-OS and the CAAC-OSshows little change from the start of electron irradiation to acumulative electron dose of 4.2×10⁸ e⁻/nm². As shown in FIG. 30, thecrystal part sizes in an nc-OS and a CAAC-OS are approximately 1.3 nmand approximately 1.8 nm, respectively, regardless of the cumulativeelectron dose. For the electron beam irradiation and TEM observation, aHitachi H-9000NAR transmission electron microscope was used. Theconditions of electron beam irradiation were as follows: theaccelerating voltage was 300 kV; the current density was 6.7×10⁵e⁻/(nm²·s); and the diameter of irradiation region was 230 nm.

In this manner, growth of the crystal part in the a-like OS is sometimesinduced by electron irradiation. In contrast, in the nc-OS and theCAAC-OS, growth of the crystal part is hardly induced by electronirradiation. Therefore, the a-like OS has an unstable structure ascompared with the nc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit contains a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of the single crystal oxide semiconductorhaving the same composition. Note that it is difficult to deposit anoxide semiconductor having a density of lower than 78% of the density ofthe single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomicratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that in the case where an oxide semiconductor having a certaincomposition does not exist in a single crystal structure, single crystaloxide semiconductors with different compositions are combined at anadequate ratio, which makes it possible to calculate density equivalentto that of a single crystal oxide semiconductor with the desiredcomposition. The density of a single crystal oxide semiconductor havingthe desired composition can be calculated using a weighted averageaccording to the combination ratio of the single crystal oxidesemiconductors with different compositions. Note that it is preferableto use as few kinds of single crystal oxide semiconductors as possibleto calculate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedlayer including two or more films of an amorphous oxide semiconductor,an a-like OS, an nc-OS, and a CAAC-OS, for example.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 5

In this embodiment, a touch panel module and electronic devices thatinclude the input/output device of one embodiment of the presentinvention are described with reference to FIG. 31, FIGS. 32A to 32H, andFIGS. 33A and 33B.

In a touch panel module 8000 illustrated in FIG. 31, a touch panel 8004connected to an FPC 8003, a frame 8009, a printed board 8010, and abattery 8011 are provided between an upper cover 8001 and a lower cover8002.

The input/output device of one embodiment of the present invention canbe used for the touch panel 8004, for example.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the size of the touchpanel 8004.

In the case of a transmissive liquid crystal element, a backlight 8007may be provided as illustrated in FIG. 31. The backlight 8007 includes alight source 8008. Note that although a structure in which the lightsource 8008 is provided over the backlight 8007 is illustrated in FIG.31, one embodiment of the present invention is not limited to thisstructure. For example, a structure in which the light source 8008 isprovided at an end portion of the backlight 8007 and a light diffusionplate is further provided may be employed. Note that the backlight 8007needs not be provided in the case where a self-luminous light-emittingelement such as an organic EL element is used or in the case where areflective panel or the like is employed.

The frame 8009 protects the touch panel 8004 and serves as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 can also serveas a radiator plate.

The printed board 8010 is provided with a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying electric power to the powersupply circuit, an external commercial power source or a power sourceusing the battery 8011 provided separately may be used. The battery 8011can be omitted in the case of using a commercial power source.

The touch panel 8004 can be additionally provided with a component suchas a polarizing plate, a retardation plate, or a prism sheet.

FIGS. 32A to 32H and FIGS. 33A and 33B illustrate electronic devices.These electronic devices can each include a housing 5000, a displayportion 5001, a speaker 5003, an LED lamp 5004, operation keys 5005(including a power switch or an operation switch), a connection terminal5006, a sensor 5007 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared rays), a microphone 5008, and the like.

FIG. 32A illustrates a mobile computer, which can include a switch 5009,an infrared port 5010, and the like in addition to the above components.FIG. 32B illustrates a portable image reproducing device provided with arecording medium (e.g., a DVD reproducing device), which can include asecond display portion 5002, a recording medium reading portion 5011,and the like in addition to the above components. FIG. 32C illustrates atelevision device, which can include a stand 5012 and the like inaddition to the above components. The television device can be operatedby an operation switch of the housing 5000 or a separate remotecontroller 5013. With operation keys of the remote controller 5013,channels and volume can be controlled, and images displayed on thedisplay portion 5001 can be controlled. The remote controller 5013 maybe provided with a display portion for displaying data output from theremote controller 5013. FIG. 32D illustrates a portable game machine,which can include the recording medium reading portion 5011 and the likein addition to the above components. FIG. 32E illustrates a digitalcamera that has a television reception function and can include anantenna 5014, a shutter button 5015, an image receiving portion 5016,and the like in addition to the above components. FIG. 32F illustrates aportable game machine, which can include the second display portion5002, the recording medium reading portion 5011, and the like inaddition to the above components. FIG. 32G illustrates a portabletelevision receiver, which can include a charger 5017 capable oftransmitting and receiving signals, and the like in addition to theabove components. FIG. 32H illustrates a wrist-watch-type informationterminal, which can include a band 5018, a clasp 5019, and the like inaddition to the above components. The display portion 5001 mounted inthe housing 5000 also serving as a bezel includes a non-rectangulardisplay region. The display portion 5001 can display an icon 5020indicating time, another icon 5021, and the like. FIG. 33A illustrates adigital signage. FIG. 33B illustrates a digital signage mounted on acylindrical pillar.

The electronic devices illustrated in FIGS. 32A to 32H and FIGS. 33A and33B can have a variety of functions, for example, a function ofdisplaying a variety of information (e.g., a still image, a movingimage, and a text image) on a display portion, a touch panel function, afunction of displaying a calendar, date, time, and the like, a functionof controlling processing with a variety of software (programs), awireless communication function, a function of being connected to avariety of computer networks with a wireless communication function, afunction of transmitting and receiving a variety of data with a wirelesscommunication function, and a function of reading a program or datastored in a recording medium and displaying the program or data on adisplay portion. Furthermore, the electronic device including aplurality of display portions can have a function of displaying imageinformation mainly on one display portion while displaying textinformation mainly on another display portion, a function of displayinga three-dimensional image by displaying images where parallax isconsidered on a plurality of display portions, or the like. Furthermore,the electronic device including an image receiving portion can have afunction of photographing a still image, a function of photographing amoving image, a function of automatically or manually correcting aphotographed image, a function of storing a photographed image in arecording medium (an external recording medium or a recording mediumincorporated in the camera), a function of displaying a photographedimage on a display portion, or the like. Note that the functions of theelectronic devices illustrated in FIGS. 32A to 32H and FIGS. 33A and 33Bare not limited thereto, and the electronic devices can have a varietyof functions.

The electronic devices in this embodiment each include a display portionfor displaying some kind of information. The input/output device of oneembodiment of the present invention can be used for the display portion.

This embodiment can be combined with any of the other embodiments asappropriate.

EXAMPLE

In this example, an input/output device of one embodiment of the presentinvention is described.

First, the specifications of the input/output device of this example aredescribed. The size was 4.3 inch (diagonal). The number of effectivepixels was 1080 (H)×1920 (V) corresponding to full high definition(FAHD). The pixel size was 49.5 μm (H)×49.5 μm (V). The external panelsize was 69.76 mm (H)×141.4 mm (V). The size of each of the displayregion and the sensor region was 53.46 mm (H)×95.04 mm (V). Theresolution was 513 ppi. A channel-etched (CE) transistor including anoxide semiconductor in a channel formation region was used.

The input/output device of this example can serve as a transmissiveliquid crystal display device. A liquid crystal element using an FFSmode was used as the display element. A color filter (CF) method wasused as the coloring method. The aperture ratio was 48.0%. The drivefrequency was 60 Hz. An analog line sequential video signal format wasused as the video signal format.

The gate driver was incorporated. A COF was used for the source driver.

A projected capacitive (mutual capacitive) sensor element was used asthe sensor element. A common electrode of the liquid crystal elementalso serves as an electrode of the sensor element. The number of sensorunits was 18 (H)×32 (V). Specifically, 32 conductive films 56 a in FIG.9A and 18 conductive films 58 in FIG. 9A were prepared. The size of onesensor unit was 2.970 mm×2.970 mm. The size of one conductive film 56 bin FIG. 9A corresponded to 30×60 pixels. The size of one conductive film56 a in FIG. 9A corresponded to 30×1080 pixels.

The one frame period shown in FIG. 8E was 16.667 ms, the writing periodwas 8.333 ms, and the two sensing periods were each 4.167 ms.

A schematic cross-sectional view of the input/output device of thisembodiment corresponds to FIG. 1B, and Embodiment 1 can be referred tofor the detail.

As the substrate 211, an approximately 0.7-mm-thick glass substrate wasused. As the substrate 261, a glass substrate with a thickness ofapproximately 0.1 mm, approximately 0.2 mm, or approximately 0.3 mm wasused. As the gate electrode 221, a stacked-layer structure of a tungstennitride film and a copper film was used. As the insulating film 213, astacked-layer structure of a silicon nitride film and a siliconoxynitride was used. For the oxide semiconductor film 223, a CAAC-IGZOthat was one of CAAC-OSs was used. The oxide semiconductor film 223 hada two-layer structure in which two layers were formed using sputteringtargets having different atomic ratios of metal elements. The totalthickness of the two layers was approximately 25 nm. The oxidesemiconductor film 223 and the oxide conductor film 227 were formedusing an In—Ga—Zn oxide. The oxide conductor film 227 had a single-layerstructure and had a thickness of approximately 100 nm. The sourceelectrode 225 a and the drain electrode 225 b had a stacked-layerstructure of a tungsten film, an aluminum film, and a titanium film. Asthe insulating film 215, a silicon oxynitride film was used. As theinsulating film 217, a silicon nitride film was used. As the insulatingfilm 219, an acrylic film was used. As each of the conductive films 251and 252, an approximately 100-nm-thick indium tin oxide film containingsilicon was used. As the insulating film 253, a silicon nitride film wasused. As the liquid crystal 249, a negative liquid crystal was used. Anapproximately 200-μm-thick polarizing film was attached to a surface ofthe substrate 261. In this example, two input/output devices werefabricated: an input/output device in which an approximately100-nm-thick APC film was used as the conductive film 255 and aninput/output device in which an approximately 200-nm-thick Ti film wasused as the conductive film 255.

FIG. 35 is a photograph showing the display state of the input/outputdevice in this example. In FIG. 35, FPCs were connected to a right sideand a top side (not shown) of the display region. For the input/outputdevice in FIG. 35, an approximately 0.3-mm-thick glass substrate wasused as the substrate 261. As the conductive film 255, an approximately100-nm-thick APC film was used. As shown in FIG. 35, with one embodimentof the present invention, an input/output device capable of favorabledisplay was able to be fabricated. Furthermore, the input/output devicein FIG. 35 had a touch sensor having a favorable detection sensitivityand was able to sense multiple points simultaneously.

Stripe-like display unevenness was observed in some images. The width ofthe stripe was almost the same as the width of the conductive film 56 a(a length in the Y direction in FIG. 9A). Thus, to make the parasiticcapacitance of the conductive films 56 a and 56 b even, the widths ofthe conductive films 56 a and 56 b were changed. The size of oneconductive film 56 b after the change corresponded to 21×60 pixels. Thesize of one conductive film 56 a after the change corresponded to39×1080 pixels. Accordingly, the resistivity of the conductive film 56 bwas changed from 1.66 kΩ to 1.19 kΩ, and the capacitance of theconductive film 56 b was changed from 534 pF to 674 pF. The resistivityof the conductive film 56 a was changed from 0.86 kΩ to 1.35 kΩ, and thecapacitance of the conductive film 56 b was changed from 930 pF to 684pF. By making the parasitic capacitance of the conductive films 56 a and56 b even, the display unevenness was reduced, so that a favorabledisplay was able to be performed. As compared to the case where a signalfor touch sensing is input to the conductive film 56 b, the displayunevenness was reduced and thus a favorable display was able to beperformed in the case where a signal for touch sensing is input to theconductive film 56 a and the conductive film 56 b alternately.

This application is based on Japanese Patent Application serial no.2015-110612 filed with Japan Patent Office on May 29, 2015, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. An input/output device comprising: a first pixelelectrode; a second pixel electrode; a first common electrode; a secondcommon electrode; a liquid crystal; a first insulating film; a secondinsulating film; and a transistor, wherein the first common electrodeserves as one electrode of a sensor element, wherein the second commonelectrode serves as the other electrode of the sensor element, whereinthe transistor comprises a first gate, a semiconductor layer over thefirst gate, and a second gate over the semiconductor layer, wherein thesemiconductor layer comprises a channel formation region that comprisesan oxide semiconductor, wherein the second gate comprises an oxideconductor, wherein the oxide conductor comprises one or more kinds ofmetal elements comprised in the oxide semiconductor, wherein the firstinsulating film is positioned over the second gate, wherein the firstpixel electrode, the second pixel electrode, the first common electrode,and the second common electrode are positioned over the first insulatingfilm, wherein the first pixel electrode overlaps with the first commonelectrode with the second insulating film positioned between the firstpixel electrode and the first common electrode, wherein the second pixelelectrode overlaps with the second common electrode with the secondinsulating film positioned between the second pixel electrode and thesecond common electrode, wherein the liquid crystal is positioned overthe first pixel electrode, the second pixel electrode, the first commonelectrode, and the second common electrode, wherein the first pixelelectrode and the second pixel electrode are apart from each other andare positioned on a same plane, wherein the first common electrode andthe second common electrode are apart from each other and are positionedon a same plane, wherein the first common electrode is located under thefirst pixel electrode, and wherein the second common electrode islocated under the second pixel electrode.
 2. The input/output deviceaccording to claim 1, further comprising a second transistor, wherein asource or a drain of one of the transistor and the second transistor iselectrically connected to the first pixel electrode, and wherein asource or a drain of the other of the transistor and the secondtransistor is electrically connected to the second pixel electrode. 3.The input/output device according to claim 1, wherein the transistor ispositioned in a driver circuit portion.
 4. The input/output deviceaccording to claim 1, wherein the second gate is electrically connectedto the first gate.
 5. The input/output device according to claim 1,wherein the first pixel electrode, the second pixel electrode, the firstcommon electrode, and the second common electrode each comprise one ormore kinds of metal elements comprised in the oxide semiconductor. 6.The input/output device according to claim 1, wherein the oxidesemiconductor, the oxide conductor, the first pixel electrode, thesecond pixel electrode, the first common electrode, and the secondcommon electrode each comprise an oxide comprising indium.
 7. Theinput/output device according to claim 1, wherein the first pixelelectrode, the second pixel electrode, the first common electrode, andthe second common electrode each have a function of transmitting visiblelight.
 8. The input/output device according to claim 1, wherein theoxide semiconductor and the oxide conductor each comprise an In-M₁-Znoxide, and wherein the M₁ is Al, Ti, Ga, Y, Zr, La, Ce, Nd, Sn, or Hf.9. The input/output device according to claim 1, further comprising afirst conductive film between the first insulating film and the firstcommon electrode, wherein the first conductive film has a lowerresistivity than the first common electrode, and wherein the firstconductive film is electrically connected to the first common electrode.10. The input/output device according to claim 9, further comprising asecond conductive film between the first insulating film and the secondcommon electrode, wherein the second conductive film has a lowerresistivity than the second common electrode, wherein the secondconductive film is electrically connected to the second commonelectrode, and wherein the first conductive film and the secondconductive film are apart from each other and are positioned on a sameplane.
 11. The input/output device according to claim 10, furthercomprising a light-blocking film, wherein the light-blocking filmoverlaps with one of the first conductive film and the second conductivefilm with the liquid crystal positioned between the light-blocking filmand the one of the first conductive film and the second conductive film.12. An electronic device comprising: the input/output device accordingto claim 1; and an antenna, a battery, a housing, a speaker, amicrophone, an operation switch, or an operation button.
 13. A devicecomprising: a first transistor; a second transistor; a first insulatingfilm over the first transistor and the second transistor; a first pixelelectrode over the first insulating film; a second pixel electrode overthe first insulating film; a first common electrode over the firstinsulating film; a second common electrode over the first insulatingfilm; a second insulating film between the first pixel electrode and thefirst common electrode and between the second pixel electrode and thesecond common electrode; a liquid crystal over the first pixelelectrode, the second pixel electrode, the first common electrode, andthe second common electrode; and an auxiliary wiring in contact with thefirst common electrode, wherein the auxiliary wiring has a lowerresistivity than the first common electrode, wherein the first commonelectrode is located under the first pixel electrode, wherein the secondcommon electrode is located under the second pixel electrode, whereinthe first common electrode serves as one electrode of a sensor element,and wherein the second common electrode serves as the other electrode ofthe sensor element.
 14. The device according to claim 13, furthercomprising: a signal line under the first insulating film.
 15. Thedevice according to claim 13, wherein each of the first transistor andthe second transistor comprises a channel formation region thatcomprises an oxide semiconductor.
 16. The device according to claim 13,wherein each of the first transistor and the second transistor comprisesa channel formation region that comprises indium, gallium and zinc. 17.A device comprising: a first transistor; a second transistor; a firstinsulating film over the first transistor and the second transistor; asecond insulating film over the first insulating film, the secondinsulating film being configured to reduce surface unevenness due to thefirst transistor and the second transistor; a first pixel electrode overthe second insulating film, the first pixel electrode being electricallyconnected to one of a source electrode and a drain electrode of thefirst transistor; a second pixel electrode over the second insulatingfilm, the second pixel electrode being electrically connected to one ofa source electrode and a drain electrode of the second transistor; afirst common electrode over the second insulating film; a second commonelectrode over the second insulating film; a third insulating filmbetween the first pixel electrode and the first common electrode andbetween the second pixel electrode and the second common electrode; anda liquid crystal over the first pixel electrode, the second pixelelectrode, the first common electrode, and the second common electrode,wherein the first common electrode serves as one electrode of a sensorelement, wherein the second common electrode serves as the otherelectrode of the sensor element, and wherein the first insulating filmand the second insulating film are located between the other of thesource electrode and the drain electrode of the first transistor and thefirst common electrode and are located between the other of the sourceelectrode and the drain electrode of the second transistor and thesecond common electrode.
 18. The device according to claim 17, whereinthe first common electrode is located under the first pixel electrode,and wherein the second common electrode is located under the secondpixel electrode.
 19. The device according to claim 17, furthercomprising: an auxiliary wiring in contact with the first commonelectrode, wherein the auxiliary wiring has a lower resistivity than thefirst common electrode.